This seems fairly simple and cheap to do if you own a gym, and especially great because gym memberships have a roster of ~3 lazy/stupid people per 1 person actually utilizing their membership to a reasonable potential. If you have new, higher-end equipment, it may be possible to interface with existing sensors and only use something like a raspi, or even just have the equipment's internals upload JSON data to a central server which either pays out a special coin or BTC. This shouldn't cost more than $50/machine and would likely be significantly cheaper.
I imagine this being extremely effective in promoting fitness, and possibly the strongest available positive reinforcement, much more so than a guy you pay to stand around telling you you're doing a good job. You offer a subscription-based model which allows users to work off their subscription fee by bettering themselves.
Example:
To join AwesomeGym, pay $150 start-up fee (which includes "handicap assessment"), then $50 or 50 FIT (Fitcoins) per month. $100/yr fee for maintenance handicapping, maybe waivable for making good progress and permitting some form of advertisement. Thus, if you earn 50FIT/mo through decent exercise, you're able to sustain membership for free (except the $150 initially, and $100/mo for handicapping maintenance).
User should be able to earn roughly 50FIT per month with above-average use... maybe 15 hours at the gym, so for each hour of work, you would want to be earning ~3.33FIT.
AG will have a low number of Handicapper Generals on staff (not necessarily essential with smart software and a biometric scan) who assesses the ability of a new user. He will test them on all FIT-compliant equipment, take the best revenue/minute results, and adjust the handicap based on that. Perhaps a fit, young person is able to generate 6FIT/hour. Since the target is 3.33FIT/hour, he would have a handicap multiplyer of .5555. Maybe an out-of-shape elderly person is unable to generate more than 1FIT/hour. That person's handicap multiplier would be 3.
Problems:
1) This absolutely does not require a decentralized solution, and decentralization would be doomed to fail since monitoring to prevent cheating is essential.
2) Gyms profit tremendously from people underutilizing their membership, with over 2/3 of membership-holders never actually going to the gym. I think a good bit of this is made up for with the high start-up fee and the genuinely good things this would likely do for people.
"Sudden cardiac arrest deaths soar as young males worked to death in marathon 28-hour exercise session"
"Gaming the system: handicap application rates soared last December"