Yeah, that will definitely slow you down even if it's intervals. Poor advice to whoever gave you that form of exercise to do...
Hmmm, my advice to you is that if you can you should work on explosive pistols with your toes flexed up so that they don't hit. Keep something around you that you can grab or fall against since you'll be balanced just on your heel. I would say lifting, but there's a good chance that you'll probably just accidentally put pressure on the toe.
Work with the upper body SHOULD be fine. I would say try to do some type of circuit like pullups, dips, pushups, etc. because if it's taxing enough it will keep your cardiovascular activity still high without compromising your legs by stimulating slow twitch muscle fiber growth. You should NOT be doing circuits with your legs. Mainly just sprinting and probably some heavy and explosive lifting in the gym.