I would focus a lot on squats and core exercises, since these are going to be what develops your powerhouse the most. The squats really really help build leg power rather quickly; if you do about 100 or more Hindu squats a day, you'd be surprised at the increase of power you'll get.
You should also do intervals, like climbing repeats, sprint intervals, etc. If you can't do these because of the weather, try getting a trainer.
Personally, I do calisthenics more than gym work. From what I've heard, both are effective but weight training gives results faster. Just be careful with the weight training, as it can cause increases in muscle mass, which will work against you...
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