this question could have a great deal of answers and a long winded one at that..........BUT i'll try to explain what i'd do.........IN BRIEF!!!!!!!!
firstly from starting training in april you have at least a full 8 months before the first of your peaks so you have plenty of time to get ready technically and physically.
i'll split this period of time into basic "phases" and i'll try to keep it very simple for you to understand, but please understand it is only basic and that because we don't know what physical shape or what training you are doing now it's incredibly difficult to advise you, BUT i'll give it a go!
PHASE 1, basic conditioning
april 1st - june 31st [12 weeks]
circuit training x 2 sessions per week,
- with varied exercise for all the body MUST include core work and infact include medi. ball ex's and try to make the ex's specific to javelin throwing. if you are blessed with good weather then a good idea is to have the 400m track as a "circuit" and at 50m intervals have a work station this could be lunges holding a medi ball above your head or a type of medi ball throw do say for 10 reps then move on by running briskly to the next stage. perform the whole circuit 3 times with 5 mins - 10 minutes recovery between. every week increase the number of reps per station or exercise by 2, once you've reached 15 or 16 reps per exercise [by the 4th week] increase the number of times you do the circuit to 4 and lower the reps back down to 10 and then again build up the reps per exercise 2 per week and so on, by the end of the 12 weeks you could be doing 5 circuits.
javelin throwing 1 x per week,
this really could be twice per week if you desire as at the end of the day only you can say whether you need to throw twice per week IF technically you aren't proficient and consistent then twice per week would be ideal, many athletes throw balls during this period BUT if you a novice thrower then javelins are the way to go as you must get as much time with the javelin in your hand as possible and learn to become comfortable with the javelin. technically wise i can't say exactly what things you should work on as there no way of seeing you UNLESS you post a video clip of course! but basically during this "hard work" phase you shouldn't think of being too technical use this phase to get the feelings of running with the javelin confidently and being rhythmic and smooth on the run way, sessions could have over 30 throws or more - depending on the intensity of the throws, first warm up as usual, then do a few short approach throws to get into it and open the neural pathways and as soon a you feel ready - this could be only after 8 - 10 throws get further back on the run way and throw using a fuller approach, so essentially you could have up to 70% of the sessions throws from a fuller approach.
if you do feel the need to only have one throwing session then you could add a further specific conditioning session.
....various footwork drills continuous for up to 1500m worth, various hurdle drills, heavy shot throws and javelin run ups [without throwing] - working on running form and X overs.
the purpose of this phase is to continually improve general fitness ans specific fitness and condition whilst maintaining technical levels.
the week plan could be....
monday - throwing session 1
wednesday - circuit session
friday - throwing session/specific conditioning
saturday - circuit session
PHASE 2, max strength/power
1st july - 31st september [12 weeks or if split 6 weeks of each]
usually this would be separate and you'd have transitional periods over lapping the two but because as i said earlier i can't tell what level you are, and again most expensed athletes will lift weights during this period.
weightlifting
they will start doing something like 70 - 75% for 5's or 6's and then build up to doing either 100% for 1's and 2's and then taper off to doing 80% - 85% for 4's and 5's, the lifts to do are best keeping to the basic power and Olympic lifts - cleans, snatch, jerks, bench press, squats [front and back] pullovers, deadlifts with perhaps one or two ancillary exercises to either strengthen weakness or build up areas prone to injury.....these could be shoulder dumbbell rotations, pulley work for ab/adductor's etc core exercises should also be done at this time.
mediball throwing
again core exercises should be done but the emphasis now should be that of utilizing power orientated exercises like soccer throws, single handed D ball throws and other seated, lying, single and two handed throws - the list of ex's you can do are endless and they can also be used in conjunction with certain technical aspects of your javelin throw to help improve that also.....a word on mediball exercises, it's my opinion that these have equal or if not more use to the javelin thrower as weightlifting does as it helps not only specifically to javelin but it also helps the athlete use and improve the elastic speed that javelin throwers must have. the great thing with mediball is that you can modify exercises to suit you and uise them in a position that's directly linked to your javelin style - try doing that with a 20kg bar filled with disc's!!! something to remember is to taper off the weights of the balls down so you start heavy and then work slowly getting them lighter so at the end of the phase you are incorporating speed...
javelin throwing
after the initial period of throwing you should now be customized to throwing again and can begin to work specifically at certain aspects of the throw, still keeping to the rule of 70% of throws from a fuller approach. finish the session with run throughs and run ups to continue to learn how to run correctly with posture and working the feet hard during the X overs.
jumps and plyo's
perhaps the best method for improving power and overall athletic ability is by performing jumps....
at first these could be just the continuation of very simple low impact exercises such as footdrills and hurdle drills akin to those performed earlier, but now's the time increase your repertoire and begin more intense jumps - the prior conditioning will give you the necessary fitness to carry out these.......they can include standing jumps, 3 bunnies, 5 bunnies, step - step - jump etc the list is endless just stick to a variation of between 5 and 10 exercises and record what results you get and aim to improve them, perhaps even include some jumps with a mediball throw attached at the end? this session can be linked to the medi ball session.
the purpose of this phase is to become more stronger/powerful in a way that's beneficial to your javelin throw and to increase javelin technical ability.
for the week plan -
monday - javelin throwing
wednesday - weightlifting
friday - javelin throwing
saturday - medi ball and jumps
PHASE 3, speed
1st october - november 31st [8 weeks]
this phase should see things done with more rest time and that goes for everything you do, as it gives your body time to recuperate so for the next throw or jump or lift you can give maximum effort.
weightlifting
weightlifting can now drop off even further and can even incorporate a kind of super set....straight after doing cleans, you can do some simple mediball throws straight up the air or after bench clap press ups and after squats box jumps......BUT they must have up to 5 minutes rest between sets. the kind of reps and sets should be something like 3/5 @ 60%
javelin throwing
javelin throwing should now be gearing towards competing and getting everything ready like run up lengths, check marks etc all the hard work has now been done and throwing should be now rhythm based with maximum effort. something which will probaba=ly help is have a mini comp with yourself at the end of the throwing session, this will help bring back the competitiveness.
sprints
various sprints can be done at varying distances but shouldn't really exceed 60m, for 4 weeks do 6 x 60m sprints then taper down to 6 x 30m sprints for the later 4 weeks......other sprints can include multi directional sprints - like sprinting towards various set cones also backwards running is good too as are reaction sprints - lie on your back, front or side and get someone to shout "GO!" and then as soon as possible get up and race towards the line.
the main emphasis on this phase is get completion ready and to become sharp not only physically but technically too.
a week plan could be...
monday - javelin throwing
wednesday - weightlifting
friday - javelin throwing
saturday - speed work
PHASE 4, competition
december 1st - march 31st
you should now be ready to peak for your first target completion and should should now cycle the power period and speed period again in 4 week blocks so it goes -
january - power phase
february - speed phase
doing it in this way will enable you to peak again in time for march.
a typical competition week could be...
monday - javelin throwing
wednesday - weightlifting/speed work
friday - rest
sat/sunday - competition.
i hope this has helped you any other queries or you don't understand anything just ask!