TC2642 ... you might be better off asking here. Plenty of opinionated folks will help you out.

Just speaking as a very late enlistee in the USMC infantry (age 29), I would suggest dropping the cycle and starting to run NOW. You need to start abusing your lower extremities immediately so that you can sort out any issues you might have with your feet/legs so that shin splints, plantar, etc. don't show up while you're in the middle of training. Running is also far more strenuous than cycling (or maybe that's just me?).

If you're going infantry, you might also want to start humping with a pack at some point.