Can I use: 'Thirty minutes away by foot'? Or how should I write this sentence properly? I want to say that: 'I will be living less than thirty minutes away, by bicycle or by (foot/walking), from the college'?
I think that “30 minutes away on foot” is more colloquially standard in American English. Otherwise, what you're saying sounds fine.
In descending order of usage and clarity... "30-minute walk from the college," "30 minutes by foot from the college," "30 minutes on foot from the college,"
"30-minute bike ride" or "30 minutes by bike." I wouldn't say "30 minutes on bike." You could use "30 minutes on a/the bike" but that seems to relate more to the time you spend on the bike vs. the trip. Like, "What did you do at the gym?" "Oh, I spent 30 minutes on the bike and then did push ups."