M, you will be fine on 25s but there is a couple of points to bear in mind:-
Between T and HB there is a number of canal "drains", where there is a depression "surfaced" with cobbles to allow flood water to drain away. These cobbles are wheel grabbers unless you are on fat tyres. PLEASE don't try to ride them, it's not worth it. Just get off and walk them. It may also be a bit slutchy here and there but just take it easy. There is a nice cafe at the park in HB (artisan, though) and there's also one just before you reach the park, on the left where there is a bike shop by the towpath. Let me know how you get on! I've actually done the route on 23s, too but it's not my preferred mode!
Peter