What is the best way to train/ cross train for roller derby?

I've started playing roller derby back in April, I am a heavier set girl and have made mountains of improvements since I've joined the league. I hold my own and can body check like no body's business. I want to become an excellent blocker and in order to even compete to play on a team I need to complete a daunting task of 25 laps in 5 minutes. I practice 2 times a week for 2 hours of straight endurance, and strength training. I am ready to make the leap to becoming a fast, agile skater. What specific training/ cross training will help me. Keep in mind that I cannot run on a treadmill due to plantars fasciitis and its inflammatory ways.. :( Thanks