I would guess some very fit, most reasonably, some not so - like civilians really, after all it's not like they a different species
To be fair though the chances of them having to chase a fairly fit person on a similar cross-country bike for quite a long distance are slim. They're either going to be chasing someone running, in which case the chase will end fairly soon - either by the crim getting caught 'cos the cop on a bike's faster, or the crim getting away due to vaulting a fence/chucking a bin in the cop's way/getting in a car or something, and the cop choosing that it's either worth risking the bike getting nicked and pursuing on foot, or it's not. In which case it just turns into a foot pursuit, which still probably doesn't last much more than 20 seconds. OR, they're going to be chasing a chav on a BMX, in which case the cop on an MTB probably won't have much difficulty catching him. To be fair though, even if they were chasing someone on a similar bike, the crim would have to be a
lot fitter and stronger, because all the cop has to do is catch up, the crim has to stay ahead, so he has to ride faster than the cop can maintain for a short burst but carry on riding faster than the cop.