Training for 2 Races at One Time

So I have the CVS Downtown 5K in Providence this weekend, and then 20 days from now I have the new Niantic 1/2 Marathon. The problem is how to get miles in for the Niantic 1/2, while not waring myself out for the 5K this weekend? I'm not certain I have a good solution, but I started today with a 4 mile run/1/3 mile swim (~6 miles total). Tomorrow, I'll try some 1200 intervals (good for both races). Then Wednesday and Thursday I'll do a run/swim and tempo (2x3Miles). The tempo is tricky - it'll be about 8 miles total - the key is getting a good run in that I can recover from in 72 hours. That gives me some decent mileage (~24 miles) without killing me for Sunday. Then I can get one good long run in next week to keep my endurance up - hopefully that will be enough.