You've got the dice stacked against you ES - Whilst I prefer Cardiff as a city to Swansea, The whole Cardiff commute is pretty grim. I did train and car to Cardiff. I can't comment re: cycling infrastructure but I know enough about routes into Cardiff to understand the worry that could be. If there are no facilities at the office why bother
My old route into work was on disused railway lines. i.e flat so no need to be Bradley Wiggins to get into work. You don't need to fit to cycle or healthy especially with e-bikes these days.
I wasn't fit to start with but recall my heart surgeons notes : He is otherwise an active man. (despite needing an urgent heart bypass op)
Even with my love of cycling a 5 day cycling commute was not for me.
P.S - Only my bottom half is 'fit' perhaps I should cycle to the GYM to do some upper body exercise, although years of working in IT may have withered north of my belly button away