Done a wee reccy of the route from the bottom of the tumble back to Cardiff.
Tumble has some steep bits for sure, don't believe what they say about it going easy after the cattle grid - it goes steeper yet.
Not much of a payoff in the downhill either. Some steep climbs between tumble top and newbridge, then s long fairly flat section to Caerphilly.
Took slight wrong turn in one way system so missed very bottom part of the hill, but at the top of the high street there is an extremely long stretch with a gradient that easily matches anything the tumble has. Save your legs for Caerphilly is my advice.
Also, the stuff about downhill all the way to Cardiff is a lie. There is plenty of down, but just as much up.
So my view on route -
first forty is flat enough bar Celtic manor bump.
Usk to tumble seemed up and down (wasn't following exact route here though)
tumble is a long grindy challenge with some steep ramps especially up second hairpin
tumble top to newbridge has short twisty downs and a fair few ups
Newbridge to Caerphilly is dual carriageway and fairly easy
Caerphilly is horrible
After Travellers Rest Pub on left there is a second fast downhill section and at the bottom has a 90 degree turn into a very narrow lane. Be careful, I guarantee there will be accidents here.
The last stretch is not a downhill rest and watch out for some VERY steep speed bumps as you come to Cardiff.