You need to ride at the pace you can maintain for the distance of the ride or slower. If you have hills, you need to know how much energy you can expend to climb them without going too deep in your energy reserves to recover before the energy reserves are next needed.
It took me 3 or 4 years of riding at least 3 times a week to figure that out. It's a moving number so as you improve and ride further or more often, it gets better. If you don't ride regularly or you just do very short rides, it gets worse.
If you use HR, then read up on finding your lactate threshold. If you have a PM, then read up on functional threshold power and training by power. Concentrate on what they recommend for distance cycling. Not what you do in sprints or short one hour rides.
https://www.trainingpeaks.com/learn/...setting-zones/
PM's were expensive when I got serious about cycling. So I used the HR method finding my LTHR. And any article I found by Joe Friel is what I read at that time. I haven't kept up with what he says about power. But now having a PM for about 3+ years, I can say that training by power is very likely better. However I haven't been doing any actual training for anything but brief periods since getting a PM. So I typically just go by the HR training that I already am familiar with for times I know that I have to get better for some planned ride that I haven't ever done.
Don't underestimate how much it does for you to just ride with someone else. It gives you both the motivation to ride more and further. As well the conversations can keep your mind off of some of the drudgery that sometimes doing the same route over and over brings with it. And with the two of you riding as a pace line, you'll marvel at the difference you see on a PM between pulling on the front or riding on the wheel.