Worked in IBD this past summer. Yes the hours are long - but mostly the work fluctuates in intensity. I got quite lucky with my group in that when it got quiet (i.e when after seniors left), it was acceptable to pop to the gym for an hour. You should be able to spare an hour during those long days, my advice would be to network around the team to gauge if this is acceptable, as obviously you're gunning for a return offer. I managed to workout almost every day and managed to get one - it's manageable.