want to do a marathon I recomend the STP. You can get in shape for it in just a few months, though a year would make it better. You have 2 options ride 2 days @ 100 miles per day or 1-200 mile day
http://cascade.org/EandR/stp/index.cfm
The training guide
http://cascade.org/EandR/stp/stp_mileage.cfm
Lots of riders, lots of fun.