I know you're far more experienced than me, but HIIT has been an absolute godsend in terms of increasing my overall speeds. My local climb is 5% for 0.94 miles with a peak gradient of 14% and when I started I couldn't even get up it in my lowest gear. Now I can average 10mph or so up there, whereas last year I was only doing 7mph up there, huge difference in power output, and I'm lighter than I was last year I do 1-2 HIIT sessions per week. 1 minute intervals with 2 minute rest periods, gets me very tired after not very long but the burn is deliciousIm just starting again to work harder as my ride mates are now dropping me on hills and doing the lions share on the front now , kudos to them but i need to start getting back to my peak of about 3 years ago when i got most of my pbs .Last nigh i did a hill session and my hamstrings were aching at the end .Mind you iam the wrong side of 50 so its getting harder to keep form .
And yes, speaking of hills and hamstrings. My average rides round here easily have 1000ft elev gain / 10 miles, sometimes more. That's likely another reason my back is under so much stress, especially as a newer rider. West Wales you cannot go more than a mile in any direction without a steep hill xD