We stayed at or near Grand Lake for a 4 or 5 days this summer. Came in thru Denver, not as bad as expected. And left via the route you are talking about. Now let me say that I am very insensitive to grades but I don't recall anything serious going out 125. Seemed a lot longer than 40 miles tho, we were heading up into Wyoming. The route is mostly empty. So keep your fuel gauge above half full.
I'll have to say that RMNP is somewhat disappointing in the things to do category when compared to other major parks. I was expecting more on the west side but it wasn't there. This is especially aggravating since you can see all those fascinating valleys that cannot be accessed unless you are a tough 40 mile backpacker. Which few of us are.
Grand Lake and the other towns along there are basically strip mall tourist traps. It's a fair drive to get into RMNP from there too.