I took around 13 months with BGS - also with a 3 hour commute, but it was on the train, which allowed plenty of reading time.
With Bristol, just trust in the Force. I can't overemphasise how good their brush-up courses are at cramming information into your head. After two weeks of Baz and his jumbo whizz-wheel, you'll be dreaming general nav questions in your sleep. Literally.
Topics that I'd struggled with and/or forgotten about at home suddenly 'clicked' when discussed in the classroom. If you put the effort in, you'll be fine.