Q1: what does "back-to-back-classes"mean?
Abra swung into the market, list in hand. She’d finished back-to-back classes, and a sports massage on a client prepping for a 5K, and polished it off with a last-minute cleaning in a rental cottage. Now she just wanted to grab what she needed and get back to Eli.
Honestly, she thought, that’s what she’d like to do for the rest of her life. Get back to Eli.Q2：what does "prepping for a 5K"mean? what does "5K"mean?Q3: in "polished it off with a last-minute cleaning in a rental cottage", what does this "it" indicate? polish what off with a last-minute cleaning?Q4：what does "a last-minute cleaning"mean?

Question

Q1: what does "back-to-back-classes"mean?
Abra swung into the market, list in hand. She’d finished back-to-back classes, and a sports massage on a client prepping for a 5K, and polished it off with a last-minute cleaning in a rental cottage. Now she just wanted to grab what she needed and get back to Eli. 
Honestly, she thought, that’s what she’d like to do for the rest of her life. Get back to Eli.Q2：what does "prepping for a 5K"mean? what does "5K"mean?Q3: in "polished it off with a last-minute cleaning in a rental cottage", what does this "it" indicate? polish what off with a last-minute cleaning?Q4：what does "a last-minute cleaning"mean?

Answer

Q1: "back-to-back" means 'in a row' - 'one after another'. back-to-back classes, back-to-back performances, back-to-back home runs.

Q2: "prepping for a 5k" preparing for a 5 kilometer (run).

Q3: 'it' refers to the to-do list on which she had written things. It was the last thing she had to do, so 'she it polished off'. She finished it with ..

Hope that answers your questions :)