I agree, sit ups aren't the main thing here. If you work on other core strengthening exercises for a while and then try a sit up, you'll be amazed and find you're much closer or can already do it. But I wouldn't advise regular sit-ups.
Lose 25lbs by 15 April (10 Mar)
Get to 100 kg (220) by 1 May (7 Apr)
Be 205 by race day - 14 July
One-derland by 1 Sept