Even though I don't have frequent access to an indoor, I ride year-round, sometimes crazily bundled up, sometimes wearing summer clothes. My horse gets furry and a little chunky, but never extremely out of shape or tubby, so I guess for me, I just continue as I have during the winter, doing mostly trot work, but I do it for longer periods of time and add short canters. If I'm really in training and conditioning, say... Starting in early March, I do interval training. Three minutes trot, four walk, three trot, four walk, over and over for the first two weeks, about three times a week.
Then, I do two walk, four trot, etc, for the next three sessions in the third week. By the fourth week, I do two walk, four trot, one canter, two trot, four walk, four trot, one canter, two trot, four walk, four trot, one canter, two trot, four walk, extending the walk periods when needed for adequate recovery time.
By the sixth week, I'm doing four minutes of canter and then three of trot work, and only giving him walk breaks as long as he needs for his breathing to recover, then back to four canter, three trot, walk break, for about four rounds.
By the end of April, I can usually do seven or eight minutes of canter work in the fields, and I'm ready to go for my events. Of course, over the summer season, he gets into better and better shape, with conditioning sessions two or three times a week, I can be into late July with nine canter, four trot, recovery time at the walk, nine canter, four trot, and cool-down. I don't push it once he's in such good form, just gradual increase. =]
And the canter work isn't galloping, though by July, the second nine-minute canter is ended with the ninth [last] minute allowing him to stretch and reach and, if he's still up for it, a hand gallop. Sometimes, when the footing is perfect, I let him really go and we do about thirty seconds of pure speed, just for fun. [He loooooves to get up and just go.]
I pretty much base my conditioning regime off of how he's feeling, and if he just can't stay cantering for a full seven minutes, I back it off to five or four and give him much longer periods of walking to rest. It all depends on the day and how he feels, how the footing in our field is, etc. If he's sluggish and doesn't seem to wanna go, I won't force the issue, we'll just work on dressage in the field instead of in the arena. =]
Riding four or five times a week, no matter what sort of riding, is always a great place to start. I guess I'm tougher than most, in the sense that even when there's three feet of snow, I'll go riding.