(well, it's probably more like 'constipation' than the other options)
My bowels have been consistently not-normal since I was born premature with mild diplegic cerebral palsy. When I was about 10 I became continent of bladder, but not of bowel, and suspect this had something to do with constipation/poor diet (i.e. with the bladder I had enough daily 'throughput' to get used to normal function, but because I would rather not eat than eat something I didn't like, I had a tendency to chronic constipation). Until I was in my mid-20s I had a tendency to get 'caught short' with unexpected bowel movements, but this no longer seems to happen in later life. Medical experts told me that if my bladder worked properly I would be able to get my bowels to work properly (and didn't have major nerve damage), but I never got quite sorted out (largely because their suggestions largely involved enemas, which don't seem to work well for me).
My tendency towards constipation is, with reasonably-good diet, not immediately affected by diet and laxatives, but directly affected by exercise (to the extent that several times in my life when I've been constipated I've taken medicine and it might take about 4 or 5 days to work, but exercise seems to work far more directly). The phrases 'slow-transit constipation' and 'low bowel motility' do seem to describe my condition rather well.
My most consistent problem is slight anal leakage (not formed stool but damp staining), and this has been constant. Enemas or having healthy/regular bowel movements do not seem to clear the problem. I've generally had this described by doctors as overflow constipation, and there have been periods in my life when this was a fair description, but it doesn't necessarily seem to work that way. The odd thing about it is that the healthier/better-behaved my bowels are the more noticeable the flow is, and exercise always increases it. For example, the week before I saw my Continence Advisor, I happened to be taking more exercise than usual, and I was passing well-formed stool once or twice a day (as compared to my normal average of once every three days, which is borderline-constipated), but this did not affect the leakage apart from making it slightly more noticeable.
My CA said she'd never happened to come across a case like mine, but when I went into that much detail she agreed with me that the problem didn't seem to be precisely constipation.
The way the 'flow' generally works is that it's more noticeable when I take exercise and for the few hours either side of having a bowel movement -- tailing off consistently the next time I sit or lie down for a few hours. I wouldn't be surprised if it's traces of stool that somehow don't get 'bound up' into the proper bowel movement, mixing with anal mucus to cause the flow that is meant to ease the passage of the bowel movement, but comes out in a less-acceptable manner.
Enemas scare me because I'm afraid of having the Spastic Panic Flinch, but also because they consistently haven't worked well for me. Nurses expect enemas to clear you out properly, and this never seems to happen for me (my most-recent experience of an enema 3 years ago involved heavy uncontrollable anal flow for the next three days, as opposed to my normal slight annoying staining).
I found somebody online who discovered she had 'incomplete bowel evacuation' from having a hospital MRI of her rectal function. Hers isn't the same because it was caused post-childbirth as opposed to being a lifelong condition, but 'leakage', 'poor sphincter control', 'embarrassing response to enemas' and especially 'the more I walk the more it leaks' seem quite familiar to me from my own experience.
I'm still at the stage of trying to understand myself better. I'll consider having Horrible Invasive Tests done in a hospital if somebody convinces me this is a good idea, but understanding the way I work (or don't) is a good start.
Remember, this is an oddity because it hasn't quite ruined my life and/or health -- I just seem to have functioned this way since I was born, consistently.