You can certainly continue to train but not doing weight bearing stuff on that leg. I know lots of folks, myself included, who did long hikes in crutches, even on rough trails so that can be one substitute for aerobic base work (not cardio). You can ride a stationary bike with one leg. You can do a rowing machine with one leg. You can use a Skierg while seated on stool. You can aqua jog (running in a pool without touch the bottom. You can swim. Any and all ot these things will help you stay active and not lose as much ground as if you do nothing. None will be as effective as hiking and running but far better than nothing.