You may be surprised at the support you might receive if you came right out and told them (in due time). Now this is not a sure thing, but it happened many times to me in situations where I thought I would hurt relationships. Before even considering my thru, saying that I needed a day off for hiking was like playing a trump card, everyone backed down, if I said I was not feeling well people questioned and tried to convince me to come in anyway. Sort of unexplainable, but my take is that I was suppose to be doing what I was doing in life and people could not oppose that. Also for the thru I feel many deeply respect that and will back off any objection.
That was my situation with hiking and the AT, not saying it will work for everyone.
Good Luck