hey Stephen
My challenge changes each week. I do 20 miles a day and obviously try to go faster than the day before. My real challenge is getting my speeds up. I say speeds as I have a rockhopper (specialised), and I carry my baby and seat on the rear end. Her weight, seat, bike is almost 40kgs of bike. I still manage to keep a reasonable 19-21 mph as an average even though we go up a nice big hill, (passing Knockhill racing track) and often go head on into wind.
My other baby is a charge plug mmmmmmmmm, anyway she is a fixed gear and have finally managed a very yummy 25.6 mph average around the same route. I have broken 30 mph on the flat and am now gunning for 30mph average over the 20 mile course. I had to change my sprocket to a 13T the other week to achieve this so may change the crank to a 46T to see if that helps as well. Downhill I have not gone over 32 mph as it is way too scary for me with my little legs turning faster than a merlin engine chasing a junker hehehehe.
I am going into a 52 mile charity ride in the summer and aim to do this in 2.5 hours about 21 mph average.