Is 72 mbps the best I can hope for on 2.4 ghz? If not, how do I increase the speed?

It is unclear how you are measuring this. I doubt that the Nordic Track itself measures the WiFi speed (could be worng but ... ). Therefore the assumption here is that you are getting slower wifi speeds in the area where the device is or you are just getting slower 2.4GHz sppeds in general everywhere. Is this what you are getting also near your router?

If it is local to the Nordic Track device perhaps you need to move a satellite due to obstructions (though this would not explain your statement about the 5GHz being great, so probably this is not the issue).

If it is general:

If you do not have it already, download something like WiFi analyzer. This could show you any 2.4GHz interference and also signal strength in db on 2.4GHz in different locations (relative to your phone of course).

If you have a number of other 2.4GHz devices going (and many devices are 2.4GHz only), try disabling them to see if you are having any congestion issues ... just to eliminate that. Are any of your devices also hotspots?

Try setting your 2.4GHz channel to a fixed channel, especially if you see interference from overlapping channels.