I really shouldn't have to spell this out, but clearly I need to, so here goes.
The CRT's Towpath Code says 'Pedestrians have priority on our towpaths so cyclists need to be ready to slow down'. No precise speed is given because it can't be - it depends entirely on the conditions, as nbt says. But as a rule of thumb, if anyone is startled or alarmed by a cyclist's speed, then they are going too fast.
It's simply a matter of commonsense and consideration for others. And at the risk of stating the blindingly obvious, cyclists and pedestrians on the towpaths are often from very different generations and backgrounds, and maybe that's part of the problem.
The fast cyclists are overwhelmingly male, young or middle aged, and fit, and they often come in groups of two or three, or sometimes more. The pedestrians, by contrast, are of all ages and both sexes: families with toddlers, kids fooling around, couples having a romantic evening stroll, elderly people who can be quite frail. The old sometimes have slow reactions, and they often have poor hearing, so they don't hear you coming. Families with young children will stand around on the towpath, getting in cyclists' way, feeding the ducks and admiring the goslings. Their dogs will wander around sniffing and cocking their legs. It's all very peaceful, and then suddenly a group of cyclists appears from nowhere, and they shoot past within inches of people, and sometimes some of them find it quite upsetting and even frightening.
All we ask is that cyclists show consideration for those who may be much younger or much older than they are, and maybe less fit and healthy as well. Is that too much to expect?