Just adding to other answers, which contain some good technical information re. calories. 

**Short version:** it's been very helpful to me, when feeling "not up to it", "not in the mood for it", or "too sore", to start with the exercise session regardless: once one start to warm up, the pulse rate goes up, and the sweat flows, all those feelings vanish like the morning fog, one feels so much better and *alive* afterwards, and really *glad* to have done the session.

Longer explanation: there are some **endocrine**-related factors related to low motivation (and perhaps depression?), which are maybe more difficult to quantify, but nevertheless important.

 * At a certain level of effort (which may differ from person to person and from exercise session to exercise session) **endorphins** come into play. I don't want to use the word "addictive" :-) but it is beneficial to start to enjoy exercise so much that it is done for its own sake.

 * In my opinion this can be improved on even further for people with an "indoors lifestyle" if the exercise can be done outside in fresh air and **sunshine**, preferably around lunchtime/noon - this improves Vitamin D levels, if done without sunscreen. (Ultraviolet radiation is needed for VitD production from cholesterol in the skin, a session of 20-40 minutes per day should suffice, but for longer exposures obviously the appropriate protective measures should be taken. The time limit nicely ties in with an adequate exercise session's duration.)

 * Exercise and sunlight also play a beneficial and not unimportant role in **serotonin** levels, again playing into the feeling of well-being. Also, [tryptophan-rich foods](https://en.wikipedia.org/wiki/Tryptophan#Dietary_sources) may help to increase serotonin levels - tryptophan seems to be absorbed better if taken with some carbs. (In general I am in favor of low carb (LCHF, banting, atkins, keto....) diets, but it does seem that *some* carbs are needed in this respect.)

 * In my personal experience for an unfit person, one would need to increase distance/effort every now and then to keep up the challenge. My practical experience confirms the 2-year estimate, although improvements can already be seen after a few months or even weeks, every time that the previous distance is not challenging enough any more. These improvements are very gratifying and motivating, besides feeling more fit and well and being able to "do things" in general life much easier.
 * **Muscle pains** can be demotivating, this can be counteracted a bit by supplements of magnesium (and zinc, and selenium). Magnesium is important in any case for people middle-aged and older, and these minerals also tie in with the whole VitD/calcium metabolism.

**Some other points to consider:**

A good **sleep routine** is also very important and beneficial in so many ways: stress relief, hormone normalization, well-being, energy levels, clarity of mind, muscle repair... The person should go to bed at the same time 7x365 (9 or 10pm latest) and sleep until rested (waking without an alarm). [No "blue" lighting 1-2 hours before bed time](https://en.wikipedia.org/wiki/Melatonin#Regulation).

A diet being propagated these days is **intermittent fasting**. Basically you only have 2 normal meals a day, late morning (say 9-10 am) and late afternoon (say 3-4pm). Without snacking. People on high-fat diets that keep one feeling satiated, may already have experienced something along those lines, just by eating only when hungry. In addition to decreasing calorie intake, those intermissions between eating help the digestive system to properly and completely digest all nutrients, while the inactive period is used for cleaning and self-repair, which optimizes digestive efficiency even further. The body gets all the necessary nutrients, "feels well-fed", and sends less craving (for carbs) signals. Modern carb foods are often much processed and refined, thus containing not many of the other needed nutrients.

It is helpful if exercise/eating/working/relaxation/sleeping can all be combined into an **integrated lifestyle**, then things tend to get done without much thought. Unfortunately the artificial or competitive nature of gyms and clubs may sometimes counteract this. I have taken up bicycling after "walking around the block" lost its challenge. (It is also easy on joints and spine, but can be done outdoors in colder weather.) To save time I combine my ride with other necessary chores like a trip to buy groceries (smaller amounts of **fresh** vegetables 2-3 times a week to help with a more healthy diet) or the library, or even to work (as many are able to do). A downside can be the dangers and exhaust fumes of traffic, if no dedicated paths are available.