Unfortunately getting miles riding on the road is the best way. i like doing walnut ->bcgb main trail, HOL, 360, courtyard, jester, yaupon, walnut
start adding in bcgb, city park, thumper, st eds, walnut as the ride becomes easier. I was slow the year I finished, but only managed to get 4 big training rides in the month before. The biggest ride was 60 miles I think.