If you have taken a week or more out of the water over the holidays this first session back is going to feel clumsy. No swimmer at any level is going to feel great jumping back in. The water will make you feel uncomfortable and what is worse the mind will play tricks on you exaggerating how bad it all feels. We ran some experiments last year and filmed a lot of the JAN week 1 one2one lessons and showed the swimmers the footage. Most were surprised it was not as bad as they imagined. Be positive, swim a few shorter sessions, work on some drills before worrying about fitness. You will not have lost much if anything <if you kept up your bike and run.> I would take off those watches, don’t worry about distance or times just enjoy some relaxed easy swimming to shake out the cobwebs.
Run through this sequence with fins or just attempt a few of the drills movements off each wall for 10m or so before resuming full stroke FC for the rest of the length. Aim for 2-3 x 30min sessions to reacquire the feel of the water. Just keep an open mind about how bad it all feels, the mind wants you back on the sofa resting and still in Xmas mode, trust me it will not be as bad as you imagine! Enjoy.