I think you can drop a little weight in the next 3 weeks. I'd try to eat sensibly with a small calorie deficit everyday, aiming to lose 1-2lbs a week. 5lbs by the 28th will help.
Try to gradually up your mileage between now and then. If your leg is up to it, put some intensity (i.e. intervals) in on your short trainer sessions.
These next 2 weekends, can you take the bike and trianer, and put in an hour in the evenings, or early mornings?
During the event, pace yourself, ride a little slower than your normal pace, eat and drink a lot, and don't be afraid to bag out before you reinjure yourself if you have to.