I know there was a thread about this 4 or 5 months back, but the search isn't working for me. Basically, are all miles good for you?

I am finding myself having the odd 20-30 minutes free in the morning and evening, so if I go on the trainer and cycle 5 miles at the start of the day and then again in the evening is that actually any benefit or do I need to have a proper structured training plan for those 20 minute bursts?

Cycling for 20-30 minutes (not miles) on the trainer in the mornings or evenings is good for you because you're active, and the recommendation is to be active at least 60 minutes a day.

The only time you need a "proper structured training plan" is if you're training for something.

I got in a lot of short rides by doing errands and store trips using the bike where possible. I know it helped me, YMMV but I would say all miles at a moderate or better pace count. No structured training plan required to improve, just ride more. Keep it enjoyable.

Depends. 5 miles on a trainer would qualify as "activity", but if you're trying to improve you're cycling that might be just enough to get your muscles warm enough to do work. Also, unsophisticated trainers don't really calibrate to road miles and speed; you can get 5 miles of virtual ride on a trainer with almost zero effort. That being said, if you enjoy doing it and it gets your day started right, go for it.

Any exercise is better than none, and a hard 20 minutes can cause very considerable adaptations. Definitely don't write these sessions off as useless just because they are short.

Cool, thanks for the responses. By the time I can actually get my bike through my house or garage and be ready to go for a ride I am having to put it away again!

In the PNW winters, maintenance and darkness also major issues for me. So I ride rollers. Even 1/2 hr. of zone 1 in the morning is great and helps a lot.

They are not if you are over training. Which is an other topic per se. Since trainers are so effective I would not consider it a stretch to wear one self out at :30 mins a day for a week let alone 2x a day. The intesity of the training is very important as well.

As a fellow Pac-Wester, I too find myself riding inside a whole lot during the winter. I mix it up, intervals on the trainer, longer spin sessions on the rollers. I feel that the two provide most of the winter maintenance I need. But I'm only half-assing a cat 5.

If you're trying to lose weight, yes. Any extra activity will burn extra calories.

Otherwise, depends on your goal.

Stay Active = You need to ride at a certain pace to get your HR up
Improve overall fitness = Ride as hard as you can for various durations during your ride ( 5s, 10s, 20s, 1 min, 2min, 5min..whatever)
Train for racing / events = Need specific training plan depending on what it is

At the moment it's just for overall fitness and keeping weight the same/losing weight. I normally vary my ride as I go, try and set a new maximum speed and then drop down, increase or decrease resistance and so on.

20 minutes is good so long as it's an intense 20 minutes

This might not answer the original post but I was thinking about good miles, bad miles.

My bad miles is when I rode too far for my conditioning level and had to suffer way too much just to get back home. It seemed to be more harmful than good.