If there is a significant market for that length of pub gig in this country, I'd be surprised. I would certainly not rule myself out for a go at it despite that. It's a worthy goal of achievement.
Being a hobby bassist is disadvantage enough. My only two gigs were 35 years ago just before I quit to put in some serious effort with my apprenticeship. I returned to bass much later as an alternative to taking up smoking again. My age and general state of decay is a disadvantage to getting match fit without a good six months of graft to make it happen.
To put in the effort, I'd need a carrot dangling in front of me. In other words, something guaranteed to make it worth the effort at the end of my fitness training. I don't see a carrot and I am not the beast of burden that some bands require their bassist to be. My hobby will never develop into a job, full stop.
I think that there are a few clubs that put on extended live musical presentations but even those don't run four hour sets by one band. I believe it still goes that events of this sort are broken up between several acts. If anyone knows of a British pub or club that does feature a band playing for three hours or more regularly please correct me.