After 10 weeks of sporadic workouts, thousands of fun/extra calories I've decided i'm finally, at last, fully recovered. And then some. This past week of slow runs and slow - but difficult - bike rides has not been all that enjoyable. I knew it would be this way, but when you tell people "we need to go slow" some do, some don't.
I always go a speed I want to go: driving, walking, running, biking or swimming -when i'm alone. Sometimes that's a very fast speed. Other times is breathtakingly slow, but it's what I want to do. I trust that my choices in this area are good.
when I used to work out primarily alone I could run a 10 min two mile and a 33 min 10K. I've never had a problem doing intervals alone. I will admit that I often cut off some of them, but I never got injured doing that.
Having said all that I get much more fit having a group to periodically train (especially bike) with. I believe you need a mix: a weekend group ride, maybe even long slow run, and a one or two workouts a week with others. Working out with my friends is always enjoyable, but running (especially running) alone is a true pleasure.
I play the piano (much more in the old days) and always found that playing in a band was a blast, but so was playing a solo piece.