Nice hilly 58 mile bike ride this morning, although my rear gear cable snapped 17 miles from home at Nomansland. This left me with the use of only the small 12-tooth cog on the rear cassette. I didn't have a screwdriver to adjust the hi-low screws :-( I only had to dismount and walk at one point - the steep hill at the top of Ryedown lane as it meets Gardeners lane (at 76.3km)
Losing the gears didn't affect my average speed - 17.9mph. If anything it increased because I lost the option to go slowly up the hills as I had to stand on the pedals to keep them turning. Good training for hill climbing :-)
I've traced the route on Bike Route Toaster so click to see where I went. This is the best site I've found for creating routes and viewing their elevation profile. Please let me know if you know of anything better.