Training logs for triathletes and runners: Why and how?
Goals and planning
A first step to starting a log is to set out your goals. Recording goals is an important element of effective goal setting (it's the "R" in S.M.A.R.T.E.R. goal setting strategy). From there you can plan your race season, putting in dates and priorities, so you can better understand how to focus your training. Diaries are also a great place to store your race results to connect what training led to what results.
All the data
Just recording the time, distance and pace of workouts will provide valuable information. Of course, with power meters and GPS watches now, you can also upload that information onto an online diary or store on a platform like Strava. Once you build a history of your training you'll be able to identify the cause and effect on what you've tried in training and the performance that followed. Replicate what worked.
Thoughts and feelings
Don't hesitate to get mushy in your diary. Research has shown that writing down your feelings, both positive and negative, can help you regulate your emotions. Research has also shown that subjective self-reported measures (mood, perceived stress) are more accurate than objective ones (heart rate, oxygen uptake) when trying to measure how you're coping with training load.
Sleep, weight and morning pulse
Aside the hard training data. metrics critical to sport performance such as hours of sleep and weight can be monitored and focused on using a log. Monitoring your pulse at rest in the morning can be an indicator of both fitness and your state of recovery. It's also important to report any illness and injuries to help connect them to any training variations or equipment changes you or your coach may have tried.
There are many ways to record your training from a simple paper daily diary to a complicated software solution like Training Peaks. I use a simple spreadsheet I've developed over the years that I share with my athletes via Google Drive.