What I would do:

high effort intervals, not sprints, but the 90%+ effort stuff working up in duration. start with 2 minute sets. you cannot do this more than every 2 days. since it's a 31 mile race, you need to do a minimum of 30 miles one day a week. if you can do 50 instead you will be much, much better off.

during your week off the bike, you really are screwed. of course you can jog for basic fitness, but it doesn't help you one bit on the bike. not at all.