My guess would be that the OP is interested in increasing average speed. Around here (PNW), that's all about climbing. 4-5 hour rides once a week, to exhaustion, hammering every hill, good pace on the flat, but a recovery pace. So Z4 and 5 on the climbs, zone 3 on the flat. Get so you can recover at that pace. Keep trying until you can do a ride like that, Be prepared to drag yourself along for the last hour or so. It'll get better, but it takes a couple years at least. "If you can still walk after the ride, you could have gone harder." Equipment is the least important thing. There are helpful supplements, but "supplements are a feather, training is the hammer." Although as I've aged, I use supplements more than I used to.
So the simplest thing to do is to find a group of endurance riders who are a little faster than you, and hang with them until you're toast. Being toast is the goal. The thing about group rides is that you immediately see what makes you faster. If you're riding near the front, you need a faster group.
That's what I did. Went from off the back to leading the group in ~7 years. I was in my late 50s-early 60s. At that age, didn't last long as group leader though.
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