Over this distance bike setup and saddle comfort will really matter so you need to get that sorted.
Don't attempt it on knobbly tyres; slicks will be much faster.
Tyre choice depends on towpath surface, there are parts of my local towpath that are large loose gravel, my slicks are near impossible to use comfortably or safely, the cobbled sections aren't much fun on rock hard 700x28s either & other parts are wet slush at times and require some skill and concentration when using slick tyres.
OP, assuming you don't know or have chance to recce the route, use google maps to get a feel for it and get in touch with the management or local 'friends of' groups to find out what you'll be riding on, it isn't necessarily going to be all pan flat road grade tarmac and on a narrow section cruddy path, it will be a big wet shock if you get it wrong.
For a self confessed novice on a likely variable quality, relatively narrow surface, next to an unprotected waterway, comfort and control should be your prime concerns rather than outright speed.