An experienced explanation please for interval trg

I've done a few 50s now and am usually out for around 30 to 40 miles on average. I don't seem to be improving with my times and someone suggested interval training. I understand that it is a period of going at it full tilt followed by a period of rest etc etc, but what periods are we talking? Some have said 2 mins full tilt followed by 3 mins rest and repeat. Others have said find a fairly level stretch of road and go at it for 20 mins, rest for 15 mins then return leg for 20 mins at full tilt again. Do they both have the same effect or am I being told two completely different things?
I ride a bike. Doesn't make me green or a tree hugger. I drive a car too.
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If it's a flat ride, for the sort of riding you are doing, try this -
Warm up, then do 3-4 lots of 10 mins steadily paced hard effort so you are breathing heavy and heart rate feels high (but not all out) with 5-10 mins recovery in between.
Alternatively do as NapD says, use the hills as an interval session, or ride the 30-40 miles at a tempo pace, so after doing it, you feel tired but not exhausted.
Best thing is just try and increase the effort you put in, and mix it up abit, if you go at the same pace/effort each time, you will plateau after a while. Also try longer distances, when you can do longer rides more comfortable, increase the pace of the shorter rides.
If you want to improve your longer-distance riding, then need to work on longer intervals. Maybe start with 10 minute intervals and build up to 20 minute ones. As Bezza said - 2 x 20 is the traditional effort for improving threshold power.
Someone else could perhaps tell you HOW to do a 20 minute interval? (describe the effort required without the use of a PowerTap, etc).
I'm still putting off starting the Time Crunched Cyclist training plan. I have only got as far as doing the 'Continuous Field Performance Test', which establishes a baseline. I bought a cheap HRM for a tenner, but I struggled to find a gentle hill that would take me 8 minutes to climb flat out.
I think part of my problem too was a habit of pushing big gears at a steady cadence instead of low gears at a high cadence. The result has been tiredness in the thighs early on preventing me pushing on where I can.