GPX Avg Speed / Virtual Partner Modifications Based on Terrain

I’m looking for something I think exists… but I may have completely imagined it.

So I have a route that I’m looking to finish within 36 hours. I can set an average speed on it (distance / 36 hours) and that’d work, sort of… but what I’m looking for is a tool that can set a more realistic average speed taking into consideration climbs / descents / flats.

I appreciate it still wont be perfect, but given I know the route is heavily backloaded with climbs, it’ll be much better than just trying to guesstimate based on overall average (I’ll be hours ahead, then I very much wont be).

Anything like that exist?

Best Bike Split would probablly work for this

Hmm, seems like I’ll need a premium membership to do goal time. Can anyone comment if it’s any good?

It seems to think I can finish in under 14 hours based on the numbers I gave it… and given the all time record for the course is 18 hours (by someone MUCH faster than me), I’m not entirely encouraged by its accuracy!

Edit - I assume I could tweak the parameters to match it to my target time and then export that?