Quite?! What would have to have been the maximum?! I averaged 68 or 69mph on a long motorway run just before lockdown, my top speed being only high-70s, but obviously a car accelerates to 70-odd in a matter of seconds rather than minutes! Plus there are a couple of brief 50 sections to consider, so my speed would have been no higher than 54-55 through these, and a brief 60 section, so no higher than 65-66.
For a maximum running speed of 125, am average of 120 would be almost impossible for a train to maintain without extremely high acceleration rates, very keen brakes and a long distance at a sustained 125mph. The GWML from Paddington-Reading is 125 for most of its run but the average non-stop booking when HSTs were new was 23 mins an average of 93.5mph considerably below the 125 ceiling.
Road traffic can maintain closer averages to its maximum speed because if the vastly quicker acceleration times and very short braking distances. Braking from 125mph to a stand at a station the driver will be cutting off the power and putting the brake it at least 1.5 miles away.
Paddington-Reading is 35.84 miles, accounting for the acceleration out of Paddington and the approach to Reading, acceleration up to 125mph and then braking for Reading I reckon that when the HSTs were new 125mph cruising was possible for about maybe 14 to 15 miles out of the ~36mi run.