I started playing when I was in high school and playing sparingly through college. I always did terrible, like 120+ terrible. I had my good shots on every hole but also had shots that would ruin a hole. So going in this winter I was determined to get better. I spent at least 3-4 nights a week at the range. Worked all on irons (full shots, half, etc). I watched a lot of videos on the different swings out there and settled on one principle. I think a major part of me not doing well was I was taking a little bit from different instructors on youtube and each one had different swing principles (ie: one plane, two plane, stack and tilt, rotary etc). and that did more harm than good for me. Once the weather got better I worked a lot on my short game at the local par 3 course.
This summer, I'm playing at least once a week and average in the mid 90's with a best of 89. I still tee off with a 5 iron, which is my 200 yard club. I recently got fitted for a 3 wood so hopefully the added distance off the tee will drive me into the 80's consistenty.
But what i've really noticed is if you damage control and make smart decisions. Breaking 100 is very doable. For me, that was realizing that hitting the fairway with a 5 iron is better than hitting from teh trees with a driver. An older golfer to just treat every hole as a par five and so far its worked.