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 yearIm 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