If you’re managing to watch the telly of use a laptop/phone while you’re on a turbo, then you’re not doing it properly, no wonder you get bored!
I get through a 60 min session of intervals quite easily, once you brake them down into blocks of effort the time passes quite quickly.
To do meaningful intervals ideally you’ll need a HRM and a turbo with power readouts.
You need to look at an average DH run and work out what sort of efforts would be best for that situation. My stuff is for XC/Cross but theory is the same. Do you often do a 2 min flat out effort whilst racing or is it a series of 20-30 seconds bursts with 10 secs of recovery/technical then repeat?
My std cross sessions are based on fairly short efforts but with minimal recovery, say, 30secs at just under max then 30 secs recovery or 20sec on 10 secs off. short but intense. I do blocks of 10 mins (10 effort/recovery efforts) then ride at recovery rate in an easy gear for 5 mins then then repeat, do 4 of those with a 15 min warm up and down before and after and you’ve filled an hour, you won’t get bored.
Just alter the time scales/intensities to suit whatever application you need. You won’t be able to do more than 60-90 secs at max repeatedly so if you need to do longer efforts, drop the intensity a little. experiment with big gear efforts at low rpm to build power or really spinning (130-140rpm) a smaller gear to develop fast legs.
Races very rarely have same distanced efforts so try fartlek training to get yourself used to different efforts ie 10 on 20 off, then 40 on 20 off etc.
If you want to develop your lacate threshold then a good session is 15/10/5. you’ll warm up them do 15 mins at your threshold, mines approx 170bpm then do 5 mins at a lower rate but not recovery, for me 145bpm, then 10/5, 5/5 etc. as you get better you can increase to 20/15/10 or 15/10/10 etc. the ist block will seem easy but it will bite towards the end of the on phase if you’re doing it right.