Can't really comment the SLSA Mal, but getting out in the water even if your just swimming is going to help with paddle fitness for sure. I have a SUP for knee & below, a longboard for knee to waist & a few shortboards for over waist

Unfortunately I don't always have time to get out in the water as much as I'd like either

If your asking on advice about these things, your probably better off hopping over to the longboarding room. Personally I think if your gonna be longboarding, you may as well have something thats an allrounder.

I paddle a SLSA racing board for fitness. I was lucky enough and got one very cheap and I use it very often. It paddles significantly faster than a shortboard but still utilizes the same muscle group so it's the next best thing to paddling a short board...and yeah, saves on the embarrassment.

I'm using mine a lot more now as I'm kayaking to rottnest in lesd than a month and need the extra bit of upper body endurance training.

^^^
Swimming (freestyle)?

I crushed L2 and had a lot of back pain. Swimming was very good for pain relief and maintaining strength and fitness...

I found that you need to set yourself a goal otherwise it can become very boring.