Aero bars are a pain in the neck - are there strengthening exercises? I've got a set of aero bars on my tourer, and now I've got the hang of them I rather like them, especially when solo and into headwinds or trying to put some distance behind me on a long straight.

A couple of weeks ago I did a rather long ride with a gentle headwind on a fast road at the beginning, so started out by using them rather a lot, and I made good use of them later too. By a rough estimate I used then for over 100km in a day, likely much more. But for a few days afterwards I had real neck ache (DOMS) in the muscles that hold my head up - I'm only a touch lower than in the drops but it seems to make a difference (my bars aren't that low to start with - the forearm pads are ~7&nbsp;cm below the saddle and they don't stand very high above the handlebar).  Yesterday's shorter but still respectable ride wasn't conducive to so much continuous use, but I could still feel those muscles getting tired - no aches today though.

While I try to get some use out of them as often as possible, I'm not happy on them in a group, and my longest regular solo ride is only 30km. So are there strengthening exercises that can be done off the bike? Perhaps something from the long time trial people.
There are a number of things you can try.

Firstly the aero position on the bike and holding that position is often related to how flexible your hips and lower back are as well as the strength of your core, so yoga, stretches of those areas, and core exercises will make it easier to hold an aero position in turn taking strain off your neck.

There are some neck specific exercises that will improve neck strength specifically those similar to what f1 drivers use to handle the G forces through fast corners.

Take an exercise band and tie the two ends to a pole or bar at height so that if you were sat down it would be at approximately head height. This will essentially give you a loop. Sit down at distance away such that when you put the exercise band on to your forehead it is tight and offering resistance.

You can then pull your forehead forward to engage the muscles. There's a link to video if my description is terrible.