I’d add that my wife is a very good hiker, and routinely goes to 14000’ (like every weekend during the summer), but still sometimes gets altitude sickness if she’s having an off day or is going to a higher elevation than that (it took her a couple of attempts to do Pico di Orizaba (18500’) because she’d get sick partway up and rather not risk it.
The point is, propensity for altitude sickness is not well correlated with general fitness level, and you don’t know until you get there.
I did the whole Cusco, Machu Picchu thing in 2019. Our hotel had coca tea available at the lobby, and I chose to go with that, as well as general acclimation. Others in our group used diamox. We had one day to hang around town before meeting our guide, and another couple of days doing excursions around the Cusco area, like waking up to Sacsayhuamán (sexy woman) above the city (at 12,000 feet!) riding bikes, and hiking around Inca ruins at Pisac - all at or around 11,000 feet. No one had any issues, but everyone was either drinking coca tea at every meal, or taking diamox.
The real test came on our 3-day Lares Trail hike, which took us up to a 15,700 ft pass. Our group stretched out a little, but generally we all were fairly close most of the day, and the differences were more to do with fitness than altitude sickness, IMHO. But again, everyone was medicating in some way with either coca tea or diamox. After the trail hike was Machu Picchu, which the altitude there was not a factor given where we were prior.
As a side note, Machu Picchu was on my “to-do” list for years. We had great weather thru the whole trip, but visiting the citadel was a highlight - it was WAY better than I imagined it to be - photos just do not do it justice. When you get there, be sure to take your time and savor it and the surroundings (Sun Gate, Inca Bridge) as much as you can. It really is incredible.
I think you’ll be fine. A couple days in Sacred Valley and then a few more in Cusco should be plenty to acclimate. You didn’t mention whether you were hiking the Inka Trail to Machu Picchu or just taking the buses up. My experience with high elevations is that the more time you spend at “high enough” elevation, the better off you will be. As Cusco is higher than Machu Picchu, then even better. Just go easy in Cusco. There are lots of cool sights to see in and around Cusco.
We did the Inka Trail a few years ago, and the tour group we used actually built in 2 days of sight-seeing/easy hiking around Cusco to help us adjust to the elevation (and for the guides to get a feel for the condition of the group members). None of us had issues with altitude sickness - even over the 13k’ passes.
A couple people in our group did encounter stomach (runs) issues, and I’ve heard of others who have encountered the same. Be very careful of the water (even on washed salads).
Bus. The hike is a multi-day trek so we took a pass on that.
I grew up at altitude and had no problem in Cusco or Machu or any of the side trips, but a few days later I got a chance to ski like a maniac at over 15k and fell asleep hard on the way back to the hotel. No other symptoms. Woke up later and felt great!
Enjoy the trip! It is indeed awsome.
It is hard to predict the effects of altitude which can ensnare healthy people. I have spent a lot of time above 8000’. I was somewhat breathy at 14000’ during exceedingly strenuous climbing, but settled quickly with puffers and pills. Fortunately the next two days were easier, there were no headaches, and descent was unnecessary.
The solution is always to go to lower elevations if in trouble.
Altitude sickness is definitely not closely related to general fitness. I was going to go hiking in Colorado with a friend who is in much better shape than i am. But the fit runner backed out due to altitude sickness, and the fat desk jockey had a lovely day alone at the garden of the gods.
When I hiked to the top of Mt Whitney, we camped at Whitney Portal (8300 ft) for a couple of days. Took a leisurely route (camped again before we got to Consultation Lake, then, camped again at Consultation lake). I still got a bit of altitude sickness – headache, nausea, and vomiting. I was worse on the western face leading to the summit, and I refused to return to camp. Made it the the summit. Vomited. Returned to Consultation lake.
We left the next morning, and as we descended I felt better with every step. Amazing. By the time we got back to the Portal, I was ready for a beer and a bag of Cheetos.
My cousin and best friend has lived in Denver all her life (with a side trip to NYC). She used to have no problem at altitude at my house. But then she got asthma. No way no how she can come to my house anymore. It’s very sad. She is my best friend.
I’ve suggested that she move to a lower elevation than Denver, but there are other issues about that.
I remember seeing the first Bussers, we were heading down from Sun Gate and a young GQ couple smelling of perfume and cologne racing up the hill on the wrong* side of the path were a bit of a surprise and a shock. I only hope that my non-cologne scents were as shocking to them 
*Hikers moving down the hill get mountain side, climbers moving up the hill get cliff side.