Just in case you don’t know already.

I put everything into a spreadsheet. Metres every day. For ordinary pool swimming, I’ll also oput down the relevant details of the session, e.g from the other day:
800WU padds, 42x100x1:40. I put in the total session time also. for sea swimming I drop in the location swim time, water temp and ambient conditions (wind sun etc)

After not too long you start to have a significant personalised database. After a few years you can become your own reference point. Check your 100m or 3000m times this time last year. What was the weather like? When exactly did I get sick? How many metres per week?

All useful for determining pattern and adjusting training should you so desire.

Remember the engineer’s maxim: anything you measure improves.

Easiest way to track is use Google Docs spreadsheet. You never lose it, add a sheet per year, sheets for charts or totals, sheets for temperature checking or a health log. Some people use a new linked full worksheet per month, with a tab for each day’s detailed training. I personally like a ton of data on the one page. Below for example is the last 6 days to today (far from the best week). I can then use this to easily generate for example the charts I’ve posted in the past.