Good call.
I've mentioned this previously, but if I cycle less than a mile from home I can join NCN 66 which will take me all the way to Wetherby.
(For the purposes of this exercise, we'll ignore the fact that there is a significantly shorter route to Wetherby, virtually all on quiet and well tarmacked country lanes).
The first bit of 66 is straight forward on street cycling. It then turns a 90 degree corner onto a rough packed bridleway which is frequently overgrown and unsuitable for a road bike. This deposits you onto a large and busy office / retail park road network.
Across this and then eyes peeled for the almost hidden turning, virtually on the roundabout that takes traffic from the office / retail park onto the motorway.
It's then a shocking poorly surfaced climb up a steep muddy slope onto another bridleway, which has a horse gate thing at the top, then round a couple of narrow turns (hope there are no horses about!) and onto the bridleway proper, which is fairly well packed earth and normally in reasonable condition, the odd few puddles excepted.
This takes you to a junction - NCN 66 turns right over a steeply arched bridge over the M1. (Were you allowed to go straight on and then left you could descend across a field and join another existing bridleway (and traffic free route) to where you're heading, but this missing link is a footpath so no bikes allowed
)
Instead you cross the bridge and descend into woods on the bridleway which is a fun ride on an MTB (if there are no peds / dogs about), but again totally unsuitable for a road bike. This brings you out onto a short stretch of cinder road, which soon becomes tarmac and you're then on the road for well over a mile, including a relatively busy section used as a rat run over the motorway (this is also part of the west Yorkshire Cycle Route, something that appears to be a local council initiative and nothing to do with Sustrans).
Eventually you reach a right turn down a farm access road (gated) which is where you'd arrive opposite if the earlier mentioned route was available to you.
Down the smoothly surfaced access road to another gate and you're then onto a rocky rutted bridleway which climbs up and is definitely a no-go for an MTB.
This levels off and continues to be rocky, rutted and potholed / puddled for some distance (still good fun on an MTB) before reaching here:
View attachment 438448
You can see how muddy it is - and this was after a fairly dry spell. You can just make out where a small stream runs to the side of the route and this regularly overflows.
The route goes through the tunnel which has an inch or two of standing water in it for much of the year, more in winter, and is unlit and curves off to one side so you need lights, no matter how sunny it is.
Out the other side the standing water is normally deeper and muddier and this continues for a couple of hundred yards beyond the tunnel in all but the driest spells.
It's then decent-ish packed earth for a good way, before turning back to a rocky / rutted surface where it drops down into Aberford.
This was the bike at the end of this section:
View attachment 438447
The route now takes to the road, through the village and then running parallel to the A1(M) for around two miles mainly uphill to Bramham Crossroads, which is a large and busy roundabout where the A64 crosses the A1(M) which you'll need to negotiate at least 4 slip roads depending on your choice of direction.
Shortly after the roundabout you have a choice of staying on the fast road, or taking to the parallel shared path. Bramham is around 2 miles north.
Once in Bramham NCN 66 continues as a shared path separated from the A1(M) by acoustic fencing, with an interesting and unavoidable dogleg on a flinty / rutted farm track, before taking to another bridleway to where the A659 crosses under the A1(M). Negotiate that roundabout safely and it's another shared tarmac path down the hill to Wetherby where after negotiating another large roundabout you're back on the road to get into the town.
So, most of the route on tarmac (and a significant amount of that on busy roads) making it a bind to be on an MTB with knobbly tyres, but linked by other sections that are effectively impassable on a road bike.
And nothing on the Sustrans website (or anywhere else that I can see) to advise what the surface will be before you get there.