Short Notice Race Fitness

Hey guys

I was asked to join an 8 hour relay race in 3 weeks. I've barely ridden yet this year (weather grr) but I want to be able to do the best I can. Any tips to maximize my fitness in the next few weeks? The laps are roughly 10kms so should I try and just ride 10kms as quick as possible? And getting out everyday? (4 man team so 90-120min rest in between laps) Anything I should do with my diet?