I guess blogging about swimming is actually the thing I know best, (better than actual swimming), having been doing it with loneswimmer.com for over five years and having turned it into the world's most popular open water swimming blog and having twice won a national Best Sport & Recreation Blog Award.
The most important, and hardest thing, about blogging, is consistency. That is, you have to keep producing content. You won't know whether you can do this until you start. Of all the people around the world who were blogging about open water when I started, no others are still producing regular content. A couple produce a post every few months though.
Swim/pool training diaries are boring. They will however bring in a small number of regular readers if you pursue that avenue.
It's probably helpful to decide what you want you blog to do and be beforehand (if possible). I decided I wanted it to be about sharing everything I knew and learned about cold open water swimming with others who were where I had been, experience wise. That's still my primary purpose. and as such it has served me very well. I never imagined I would become known for the further specialist subject of cold water swimming.
Finding time to write is often very very difficult. Finding ideas equally or more so. I always compare swim blogging to marathon swimming. The most important thing, is to. Just. Keep. Going. How you find ideas is for each of us to discover and rediscover. Keep a list of potential ideas. Write down vaguely interesting titles or sentences. They may turn into articles later, or spur a further idea.
Find your own voice. Don't write to what others expect, write to what you want. Don't be embarrassed. Don't worry about the quality. Treat the first year as learning. Assume you will make mistakes.
Pay attention to SEO from the start. I paid no attention to it for two years and very little for the next two years. It's only recently I've been reading up on it and the simple mistakes I was making for years, especially with titles and tagging.
Make sure you write something humourous at least occasionally. I'm not spontaneously funny. Writing humour is hard. It's also hugely effective and (personally at least) very rewarding. Every time I find an idea though, I think it'll never happen again. I wrote an off the cuff humourous article on a cold water temperature scale 3 or 4 years ago. It's still very popular and strangely, it's also now referred to by many swimmers around the world as a guiding scale for open water. Simply collecting stuff off the 'net though from other locations is mere Pinterest or Swimswam blogspam. Originality is essential.
Linking to and from other blogs is still the most important way to get readers, same as it ever was.
Be ambitious. This is one of the most important. I have learned that as popular blog author, I can change or at least affect the entire world of my sport, always I hope for its betterment. I ended up co-writing the first ever global set of marathon swimming rules and co-founded the only marathon swimming forum. I'm just an average swimmer in the middle of nowhere, Ireland. Single articles I've written, such as the guiding principle How the hell? The pint is, don't be afraid to tackle things that you thing others will know more about. Knowledge is not subtractional, its additional. We can all add more, no matter our own level. As a returning swimmer, you will have a huge potential audience.
Use multimedia. People reading blogs don't look at many video links. They don't follow many links either. They do look at and require blog images. Those three short sentences took me a year to learn.
Here's my answer to your specific first question. I might read your blog regularly. At first. I myself need to know someone is committed to regular blog updates. To see if I think it's going to be a real thing. (Most blogs don't survive that period. Few make to 6 months, less to a year, less again to 2 years etc). If I like it at that point, and you've written a few good or interesting articles, especially if you've written a funny article or one I could learn from, then that would change and I would link to it.
However, I'm especially currently interested in your proposed subject area, so if you go ahead, let me know.
I'm happy to answer any other questions I can about swim blogging. I can get as technical & detailed as anyone wants like.
I wish we had more people submitting good blog articles on Swimmit (not blogspam). it's not easy, but it's incredibly fulfilling when it works.