I started training for long distance running-marathon in January.I started with a dismal mile time of over 8 minutes,then 7:30,then 7:10,then 6:50 and now I can run in 6:10.It took three months for this improvement.I want to know whether this improvement is good or not.Also I want to know how long will it take for me to improve this timing to 5:00 or less.Also I want to maintain this mile time for that is 6 minutes for 15 miles and then decrease this timing to about 5:00 for the same 15 miles.How long will it take and how should I change my training plans-presently I run roughly about 50-60 miles a week?