I imagine less than half one's time actually being on the move is pretty typical. For instance, here's my journey into work:
Raw data:
Walking from my house to the station 13
Waiting for the train 5
On a moving train 40
Intermediate stops 3
Changing trains 4
Walking from the station to my office 8
Being early for work/desk breakfast 24
(for a total of 97 minutes)
Interestingly, Google predicts enough traffic to think that driving from my house to my office would take 75 minutes to get there for my start time (going up to 85 on the way back). So once one adds in finding a street just far enough out of town not to cost some extortionate daily rate in parking, then walking back in, driving would be a very slow and not very practical option. (But when having a nice commute by train is a factor in where one lives and works, this was probably going to be an inevitable consequence.)
As for improving bus journey times, I don't know Bath well at all, but if it's anything like Cardiff, the key issues will be bottlenecks on the outskirts of the city centre. For instance, Cardiff Council could improve bus journey times no end by banning cars from all of the bridges over the Taff except the A4232 and the M4 (and any council that tried this would immediately lose the next election).
The other thing that makes buses really unattractive is the way that so many British cities have radial routes that get into the centre and terminate, rather than continuing out the other side. This gets particularly ridiculous when one throws in pedestrian zones, so that buses will tend to terminate from both sides on the near side of the centre (forcing a long walk to interchange) or on silly one-way loops with a massive layover half-way round (a recipe for circuitous and slow journeys – I don't know why bus companies don't understand the basic point that most passengers do not want to travel in circles).