Not bad effort for two weeks off!
Well today's ride was absolutely shocking 30.3 miles in 2:30:51 @ avg sp of 12.1 (thats the bad bit) elevation gain of 1808 ft (thats 59.67 ft per mile)
Anyway I have worked out a rough formula for difficulty of ride, if you take the height climbed per mile i.e. 59.67 multiply by the avg speed 12.1 you get a figure of 719.12 then multiply by the distance of the ride i.e. 30.3 divide that by 1000 and you end up with 21.79 difficulty.
a short run I did on Wed of 8.22 m had about the same climb rate
so 12.3m.p.h x 59.49 ft climbed x 8.22 miles / 1000 = 6 difficulty.
another longer ride but less climbing
(14.4*34.55*52.5)/1000=26.05 difficulty (31/08/2012 date of ride)
yesterday
(13.1*51.51*35.7)/1000=26.93 difficulty
So yesterdays ride was as difficult as the ride on the 31st of Aug but 16.8 miles less.
another ride on the 26/07/12
(12.6*21.5*46.7)/1000 = 12.65
So (AvgSp x height climbed per unit distance x distance) / 1000
you could use either metric or imperial.