From my personal experience average gym cycle trainer data is junk.
Your MTB efforts should be closer to real (very close in fact) but looking at your strava profile it does not seems like you are anywhere close to 300W.
Also going from 70 to 62 kg is like dropping your body fat from 20% to 10% - unless you tried that before you will be very surprised how difficult is that.
That is assuming you are at 20% (or more) body fat right now - on your strava picture you look pretty fit already.
I went from mid 15% to low 14 over last year (lost about a kilo or so) and seriously doubt I can ever get close to that "ideal" 10%.
If you that serious you can do hydrostatic weighing (around $50 in our area) or dexa scan (~100$) to get your accurate current body fat and start from there.
Try to do proper FTP test first - it is free and fun
