As a function of forest fires and wanting to meet a weekly training schedule, I have done more miles than I would like in a respirator - simple N95 as well as a dual P95 cartridge FM. I have also done a fair amount of reading on the subject.
Couple thoughts -
A) It does not simulate altitude. Altitude would mean less O2 per given unit of air and a fixed ratio of CO2.
B) It does represent CO2 build up / over saturation
C) You’re going to tax your body. Make sure you’re in shape for that. A CO2 build up plus exertion can be a recipe for a heart attack. From what I could gather / guesstimate it makes it @ 25% harder from the standpoint of CO2 alone - add heat or other stress - you’re going to start to really tax your body.
D) It is uncomfortable and annoying as shit - the weight of a dual filter unit, sweating, blowing goo in your mask, 90+ degrees and smoke. You know it is working if you hear the cartridges ‘click’ and the flow of resistance change with each direction of your breathing. I was good for 25 - 30 miles a week, but I was really happy when the rains came and the fires were extinguished for the year. It was not uncommon to have a slight headache for a bit after the run. I did get behind the curve once while masked and got heat stroke - wasn’t fun, but self recovery was attained after a few hours.
Definitely read about the effects of CO2 on the body and take a little caution with this; if your not in pretty good shape - ease into it.
If your body is trying to suck down air, its because it needs the oxygen. Restricting that intake is not the best idea for long periods of time or for intense workouts.
From my experience, Crossfit WODs to a great job at restricting my air flow, lol. I'm not a Crossfit dude, but I went through a LE academy ran by a certified koolaid chugger. Those damn assault bikes...
There is only one reason to train with a "mask" and that is if you have to work/survive with one. This type of training, including altitude chamber rides for pilots, does not make you any fitter physically; you learn to recognize and adapt to your reduced performance at the risk of pushing yourself over the edge. That's why they have medical personnel at the ready during this "training".
The human body needs weeks to adapt to continuous, mild hypoxia by temporarily increasing the red blood cell count.
OTOH, there is no natural way for our body to reduce the risk of a CO2 hit.
(You may want to discuss this with some one who dives, preferably with a rebreather.)
A while back I read an article detailing the real science behind altitude masks. In short, the science isn’t supportive of the claims made by people who use altitude masks. It takes a minimum of 2 weeks for your body to acclimate to altitude. Altitude is based on environmantals that cannot be artificually replicated with a simple mask. As was noted above, there are biological processes that occur to increase RBC production and whatnot. The only way to artificially increase your hemoglobin and hematocrit levels is by TRT or steroids. If the reason to get a mask is to strengthen your lungs, then you should be working harder while doing cardio instead of buying into the BS that you can somehow game the system.