Last time I tried, Zwift didn’t kick me off, after reaching 25km in a month. I didn’t get experience points or whatever it’s called, but I don’t care about the gamification side of Zwift.
There some long climbs on RGT but as you said it’s hit or miss if free.
Thank you !!!
Yeah, I seem to remember reading that you don’t get booted out once you burn that 25km. But I expect a person would not be able to log in and ride a 2nd time once you close the app and reload it. I have never tested, so that’s a guess, but I’d be surprised if that would work.
So, you either get 1 ride per month (25km + whatever you continue to do at that time), or leave the app open all month to ride unlimited? 
I’ve never left the Zwift app open.
Maybe they have a gap in their programming then, because unlimited free use is not in their plan.
Q: What if I want to try Zwift again—but am not ready to become a member just yet?
A: No worries, we’ve got you covered. On the 1st of every month, we’ll give you another chance to try Zwift with another 25km (15.5mi).
“try Zwift” means the full Zwift experience. I’m not getting the full Zwift experience after 25km is completed. I haven’t tried it since last winter, but it worked before. Not sure if that would limit my ability to hit the long climb.
Could be a loophole hiding there but I’ve not heard it mentioned anywhere.
Of course, simulating struggling uphill is hardly the best way to train. 
Best? Not if that’s all you do.
But including relative efforts like the expected ones sure have a place in the overall picture of a person’s training plan. Hence the reason my first reply was a search that included results discussing full training plans along with more specific climb-focused work.
Whoosh is free and has several long climbs.
RGT has the (think it is subscriber (paid) specific) option of creating your own routes/roads, to be used in their Magic Roads section.
Allows one to create a route on a third party platform of anywhere in the world, upload it to RGT and ride/train on it anytime you like.