As others have said there are many different options and you just have to find what works best for you.
Just riding more miles is not necessarily going to help. After a while they just become junk miles so you need balance in your program.
What I've added in the past year has made more of a difference then anything else I've done over the last 10 years.
Added spin classes, weekly over the winter and then a couple a month during riding season. Definitely helped my hill climbing. During training rides I've eliminated as much as possible using my small front ring.
Makes you work harder but it definitely pays off. My average ride is 54 miles with 2,950 feet in elevation. The biggest single difference for me is when I started using the Glute machine 3 months ago. Not every gym has one but great for building strength and endurance as long as you make sure you doing enough reps to give your legs a good workout. And of course don't forget your core.
