@rheerios, I'm a year older than you ... so you're my age ... So basically all people our age have to start slowly when getting back into shape--joint replacement or not! .... That's sorta what I've learned painfully by foolishing doing too much .. repeatedly and paying for it later.
These days I find that I get the clearest feedback from my body ... not just the day after ... but often two days after ... I will be walking around tired ... just feeling blah ... Often it's when I've worked out too hard ... on weights or on cardio ...
At first I will think I'll bummed or depressed about something ... and then I sorta do an audit of my life ... and it always comes back to having overexercised a day or two previously ... though it did not feel exhausting at the time.
God, I once lifted weights about a month ago ... at the same time I was cutting calories. OMG. My body screamed a day later ... with fatigue!
So I think the same rule applies after joint replacement ... exercise ... see how you feel, see how your leg feels, a day later ... two days later ... My thinking is that it's best when getting into shape again ... to work out frequently at shorter time and lower intensity than it is to work out hard on a sporadic basis.
But your body will LOUDLY SHOUTINGLY tell you the pace you need to work at ... Give a day or so ... Unfortunately it seems that it is impossible to not overdo it occasionally because you can feel fine during the exercise and immediately afterwards ... and then the pain, stiffness, fatigue ... occur later.
Still I would check with the surgeon ... One reason I chose my surgeon is that he's OK with aggressive exercise, including running ... I wanted the placebo effect of his endorsement of aggressive activity. So you might get a nice little placebo effect to hear the surgeon's office endorse what you're doing. Then you can just concentrate on the exercise and drop the worry that you might not be doing it right.