The fear of holding people up keeps a lot of people running alone
Somebody asks if you want to come along. A friend who has started running, or a local group that meets on Tuesday evenings and says all abilities welcome on its page. You say that sounds good. Then you find a reason, and the reason you give them is not the real one.
The real one is that you can already picture the evening: you fifteen metres back, everybody else slowing without discussing it, somebody glancing over a shoulder to see where you have got to, and then a short kind conversation at the end about how they were having an easy week anyway.
What you are actually afraid of
It is rarely the distance. Most people weighing this up have a fair idea of what they can cover, and if they do not, one evening settles the question. What stops them is the arithmetic of somebody else's time. You are willing to have a hard, ugly half hour on your own. You are not willing to hand that half hour to four other people and make them carry it.
That instinct is a decent one. It is the same instinct that has you apologising when somebody else bumps into you. It is also, in this particular case, costing you the one thing that would make the running easier to keep up.
Notice how specific the fear is. It does not attach itself to the miles. It attaches itself to the two minutes afterwards, standing about while somebody is being generous about how you got on.
Any group has had every speed through it
A group that has been going for more than a season has seen the whole range: people who turned up once and never came back, people who returned after a decade away and could not manage the first loop. None of it surprises them. It is the ordinary condition of the thing.
So when somebody asks what pace suits you, they are sorting out logistics. They want to know which group you should start with, or whether tonight needs to be the flat route. You hear it as an exam because you have been marking yourself for months. Nobody else standing there is marking it.
There is also something that people new to this tend not to expect. The runners who cover the most ground spend most of their week going gently. A slow evening is usually the session they should be doing anyway, and company is the most reliable way of making themselves hold to it. You are not the reason the pace is easy, and a few of them will be quietly glad of the excuse.
What the company is actually doing
The useful part happens before you leave the house. If you agreed on Sunday to meet somebody at seven, then at half six on Tuesday, seven is no longer available for discussion. The decision was made two days ago by a version of you who was not tired, and the arrangement holds it in place while the tired version gets changed. That is most of what running with other people gives you, and it is worth more than any encouragement anybody offers you mid run.
The rest of it is that half an hour of talking passes at a completely different rate from half an hour of listening to your own breathing and working out how much is left. Runs you would find heavy alone turn out unremarkable in company, and you finish them faintly surprised at yourself.
None of which means every run should be social. For plenty of people the running is the one part of the day when nobody wants anything from them, and handing it to a group would spoil it. If you come home from a Tuesday evening having spent the whole hour managing how you came across, that was a worse hour than the one you would have had by yourself. People run alone for years because they prefer it, and that is a complete answer on its own.
How to make it less of a test
Say it first. Before you set off, in plain words: I am slow, I take walking breaks, I have been out three times this month. Saying it out loud removes the thing you would otherwise spend the entire route bracing for, which is the moment somebody finds out. There is nothing left to find out once you have already told them.
Go with one person before you go with twelve. Pick a route where you can turn back, or a loop that passes the start, so that leaving early is an ordinary thing to do rather than an announcement. And let the first one be awkward. It usually is, in the way that any first evening among people you do not know is awkward, and most of that has nothing to do with running.
The evening you keep declining is a duller thing than the one in your head. Somebody picks a route. There is some talking, a little of it about running. At some point you go quiet on a hill, and so does everyone else, for their own reasons. Then it is over, and the shoulder glance you spent a week dreading either never came or meant nothing, and you are the one who got out on an evening you would otherwise have talked yourself out of.
Written to encourage, not to diagnose. If something hurts or feels wrong, please speak to a doctor.