you can plan a small microcomputer, something like:
a microcontroller: 8051 family as you liked
a data display: maybe a common alphanumeric LCD, or some simple 7-segment displays.... (or something more sophisticated)
a data input: some push buttons or even a small keypad (3x3 seems enough) for gender selection, and weight input
a process sensor: mmm you need a rotary encoder. it will give pulses for each turn, and you attach at some part of your treatmill... as faster it will go, it will generate more and more pulses, if you count the pulses, it will be directly proportional to your total length... if you count the pulses every seccond it will give the speed at the given seccond.... (of course by mechanican design you need to apply a constant factor to get your speed in m/s and not in pulses/s)
as your treadmill doesn't move, i wouldn't plan for a battery-powered circuit and even a permanent memory for the data. just a regulated 5V and the internal RAM of the micro will do the job.
post here if you need further help. but don't expect that anyone will do your job, just help.