I agree with Steve - general health is most important, i.e. obtain appropriate vaccines & any medications you require. As to fitness, I think a base is useful if you expect strenuous riding in such conditions as mud, sand, extreme heat or cold and very high altitude; if you're running quite a bit now then you'll be in better shape than 99% of overland bikers. Keep in mind that you will get 'bike-fit' after the first few days - the sore bum goes away, hands don't get numb easily, the right muscles stretch a bit and you'll find it no strain to ride long stretches. Your body will adapt to the environment it's in and your technical skill will improve to make riding easier overall.
IMHO jogging is the best way to keep up one's fitness while on the road, so pack the trainers. Continuing a weights routine is unrealistic.