I changed it a bit. Journal is created when session starts (this is first time [Start] was clicked), and it is valid till the end of the session (with the app). No matter how many times training was stopped/started. Session is closed when the app exits.
After recovery, previous journal is restored (with all the data from it), and one can continue the previous session: it might be used to resume training in case of power loss (or just application was lost). But [Recovery] must be done prior [Start].
For testing purposes.. it would be nice to add [Start new session] option to the menu. But in fact it is not needed for "normal" trainings (and nobody will start new training just after another).