I've been running for about six months and have done a few easy 15k runs lately. I find them too boring to be enjoyable, but otherwise not particularly challenging. However, I'd like to be able to do a longer run and have the confidence to know I can do it if I want to. Would it be practical to do an easy half marathon say once a month, but otherwise to restrict my runs to 10k (the distance I enjoy most).

I'm mostly running for weight loss, fitness and enjoyment, rather than performance (I'm not interested in racing, except an occasional park run), so I am primarily concerned about injury risk (I am in my 50s, no longer overweight and in reasonably good health) and any other potential disadvantages I might not be aware of.

I am running three times a week at the moment (total 25-30 km), but also go to the gym and swim regularly, so there isn't much scope for increasing the number of runs.