Someone else will have to answer about the average speed and range achievable with a small engine: I do know that the full size engine with trailer that once did the 30 mile round trip to Bath delivering coal from a local colliery took 2 days to do it, 3 if it was wet! I'm guessing the reason the journey took so long was that the engine moved at walking pace – less than 4mph, had to stop frequently to bunker fuel (from the trailer), take on large quantities of water, and to rest the crew. I doubt it moved at night.
On the road many things conspire to reduce average speed. One is the number of stops and restarts due to junctions and queues encountered on the journey. Running the engine down the cycle track on a dismantled railway will be much less trouble than traversing a series of busy main roads with an engine that doesn't accelerate well and has a low top-speed. (Driving to work in a car my expected commute time was nearly tripled in practice by traffic controls and other road users. Even worse in bad weather, or if there were road-works or an accident.)
Another issue is what you need to do when the engine breaks down halfway.
I think the logistics need careful planning, and suspect the engine would have to be followed by a support vehicle during a long run.
Another issue. At a show I love traction engines. On a busy main road, when I'm in a hurry to get from A to B, chaps pratting around and getting in my way with their caravans, horses, push-bikes, line-painting, drain cleaning, temporary lights, and theodolites get short shrift. Your innocent enjoyment of a fine traction engine out for a run on the open road might not be appreciated by the angry victims of the jam behind you. Even though I'm the most tolerant person I know, beware. Delay me 5 minutes and I'll tip your engine into a ditch and organise a lynch mob.
Dave
Edited By SillyOldDuffer on 05/10/2018 12:36:24