If this is an undergraduate level research paper, then knock yourself out.
If it is heading towards publication, then you'll need a LOT of data.
One of the confounding variables is that the max heart rate, and perhaps the resting heart rate is supposed to decrease with age. So, you may need to at least group your subjects by age.
Unfortunately I don't have the data yet. I'm trying to convince my devices to talk to each other
I was stunned to find that my resting HR seems to have decreased by about 10 BPM over the last couple of years. My last "max" was checked about 15 years ago by hand, so I still need to check again, and never did a threshold training determination.
If you have time, perhaps one source of data would be to go to an army base, and work with army recruits. Get data on admission, then follow them for a year or so as they go through progressive training. Perhaps there would be other groups who would go through significant changes in their ordinary training routines.
It is possible that they would already have the longitudinal data.
The advantage of a longitudinal study following individuals through time is that you could correlate their pre-training and post-training data on an individual basis, and thus significantly reduce some of your data variation.