Forget your old PRs. Your training cannot be focused on reliving the "glory days" trying to throw 405 back on the bar for squats for reps, you are now forging a brand new and better foundation. Whether it is a return to aerobic training, strength training, or a combination a new baseline has to be built. Just after 2-4 weeks off from aerobic training, there is a drop off in VO2, and after 3 weeks or more strength decreases are noticeable. What this means in a practical sense when returning to training you cannot go off of old PRs as a reference point. We have to all set our egos aside and be willing to go a little lighter, slower, and a little more conscious of what we are doing than before our time off. This means increasing volume slowly before increasing intensity too quickly. From an aerobic perspective utilizing an introductory running plan (a couch to 10K program for example) may be beneficial for the first month slowly building back up miles on the feet and cardiopulmonary system. From a strength training perspective, I highly suggest doing simply one working set of each exercise between 12-15 reps for the first couple weeks back increasing weight slowly and warming up properly. This will most likely be done at an RPE (Rate of Perceived Exertion) 5-6 when first returning. This will feel slow at first for many of the hard chargers out there but the slow build is far preferable to burning out or re-injuring yourself trust me from personal experience. All of this reintroduction to training is based on one key concept and that is being aware of signs of functional overreaching or physical and mental signs of possibly pushing a little too hard too quickly.