Personally, I'd start by breaking the game into general "stages". For example, level 1-5 is a quick introduction to hook new players, 5-10 is challenging but not hard and content rich. 10-15 is hard and grind heavy, intended for the smaller group of players who take it very seriously. 15-20 is very challenging and very stretched out to give addicts endgame content.
Then I'd give rough hour guides to each: first stage is 10 hours, next is 40 hours, next is 150 hours, next is 1,000.
Then I'd estimate how much of each stage is combat driven vs. story, crafting, socializing, moving etc. Maybe (35%,25%,40%,45%).
Then I'd estimate how long a fight should last. I might go (1 minute, 3 minutes, 5 minutes, 20 minutes). Battles get more complicated as we go and in the end game are very much a grind.
Those numbers give me: 210 fights to level 5, an additional 200 fights to level 10, an additional 900 fights to level 15 and an additional 1,350 fights to level 20.
Then I'd divide the fights per stage into fights per level making the early levels faster and the later levels slower. For example (30/40/60/80) for your first 4 levelups.
Then you have to decide how quickly power scales. If it scales too slow, players can grind easy monsters. If it scales too hard, there's a risk of players rocketing through the levels if they can beat higher level monsters. As an example, let's use the level*level*5 as the monster value (5 at level 1, 1805 at level 19).
Then your first 4 levels are:
level 1: 0 xp
level 2: 150 xp
level 3: 950 xp
level 4: 3,650 xp
level 5: 10,050 xp
If you penalize player xp for dying, you need to factor in expected deaths.
Changing any of the assumptions gives you a different calculation. During testing (and if possible, after deployment) measure and see if you were wrong: do fights go faster? Do players spend more time traveling than you thought? Is the monster scaling too shallow? Is there too much content in stage 2 and you need to slow down progress through it? Just make the relevant adjustments and recalculate.