The most recent studies I have read indicate that there are people who get into shape easily and people who struggle to stay in shape even when exercising regularly. That matches with what I observe in our family.
Every summer I do a two day-150 mile bike ride. I always intend to work out ahead of time, but am not so good about putting those intentions into action, and usually end up riding several 20 mile jaunts leading up to the ride - but not the 100s of miles I should. This year was the worst - on the day of the ride I had not been on my bike in 11 months (and I was 50 lbs overweight - just into the obese range using the BMI scale).
The first day's ride (75 miles) was no more difficult than any other first day (easier than many because of perfect riding conditions). It took me about the same length of time as it always does. Day two was a bear - but completely beyond my control. The weather included a steady light rain and a temperature of around 50 degrees. Because I had to pack for the ride a week in advance when it was in the 80s, I was dressed for 80-90 degree weather. I decided it was insane to continue past the first rest stop (at 12 miles), and the ride was formally called within an hour due to the number of riders experiencing hyporthermia. Had it not been for the weather, I would have had no difficulties finishing the ride.
I have always known that I get back into shape easily - but finishing the first day's ride without any hint of overexertion (no muscle soreness, heartbeat that returned to below 100 within 5 minutes after I hopped off my bike, no more fatigue than I usually experience after relativel continuous exercise over a period of hours) even surprised me. I went into the ride fully expecting that I might not finish one or both days because of my own physical limitations.
My spouse, on the other hand, after exercising regularly (2-3 days a week) for 45 minutes to an hour over a period of a year is now finally able to keep up with me on a 10-20 mile bike ride without asking me to slow down (from 10-13 mph) or stop for a break.
The goals you have set for yourself do not look to me like difficult ones (except that I hate to run). Given my exercise regime (or lack thereof) in recent years, I do not deserve to have it that easy - but I do. On the other hand, I know that even in the good shape my spouse is in now - having really worked hard to get there - it would still be an impossible goal. Life's not fair.
How your body has responded in the past (either to maintaining fitness - or to getting back into fitness after a break) will be your best guide - and that is nothing any of us can tell you. Good luck - and kudos for setting healthy goals for yourself!