Overworld data
The overworld path pointer array starts at 0x6163A.
From there, you start reading 8-byte 'crosses'. They're called crosses because they store the info of all the four points of the compass.
Below are the few first 'crosses'.
------+-------------------------------+
6163A | FF FF | FF FF | FF FF | FF FF |
61642 | A1 5A | AE 5A | B8 5A | FF FF |
6164A |	FF FF | FF FF | FF FF | C3 5A |
61652 |	12 5B | D8 5A | FA 5A | CD 5A |
6165A |	FF FF | 18 5B | FF FF | FF FF |
61662 |	FF FF | 1E 5B | FF FF | FF FF |
------| Right | Left  |  Up   | Down  |
------+-------------------------------+

	Let's observe cross #2, at 0x61642. The first word is A1 5A; that is our pointer.
	First you swap the bytes and get 5A A1. Let's interpret 5A A1 as XA BC. The offset
	where the path instructions are located is found out through the following formula:
		0x5C000 + (0x1000 * X) + ABC
	Which in this case would be:
		0x5C000 + (0x1000 * 5) + AA1 = 0x61AA1
	
	So let's actually go to 0x61AA1 (the instructions for pushing right on cross #2: 
	01 01 08 08 08 01 01 01 01 01 01 00 13

These are the instructions. They are like small, one-byte commands to Mario of where to go and how.
The first nibble tells you HOW to move, and the second WHERE.
	First nibble:
		0 = Walk
		1 = Half-walk
		2 = Tiny
		3 = Tiny (also)
		4 = Invisible
		5 = Invisible
		6 = Invisible
		7 = Invisible
		8 = Climb
		9 = Walk
		A = Walk (choppy)
		B = Tiny
		C = Invisible
		D = Invisible
		E = Invisible
		F = Invisible half-walk
			Now there's a lot of invisible stuff there,
			you might start to think that they're
			'invisible climbing' or something like that,
			but that would mean that one bit means a property,
			but it doesn't. It might look like it's like that,
			but no, they're not bitwise.
			
	Second nibble (this one is bitwise):
		Bit 0 = Right
		Bit 1 = Left
		Bit 2 = Up
		Bit 3 = Down
			-Your crazy combinations such as Up-Down and Right-Left are in favor of Up and Right.
			-Diagonal movement very possible.
			-No bits set means Mario being stationary. Can't think of any situation where to use this.
			-Whenever there's a 00 (stop) it means that the next byte is the destination cross.

So 01 01 08 08 08 01 01 01 01 01 01 00 13 would mean the following;
Walk right two tiles -> walk down three tiles -> walk right six tiles -> stop / now you're at cross #13.


But how will we know what cross leads to what level?
There is a byte array starting from 0x3C218 that links to the 'crosses'.
The first few bytes are 00 FE FE FE 19 11
Let's put them next to the cross array. ('LB' means Level Byte and 'LBO' Level byte offset):

------+-------------------------------+------------+------------- -
6163A | FF FF | FF FF | FF FF | FF FF /	00 | 3C218 | 'Mushroom' Intro Level	
61642 | A1 5A | AE 5A | B8 5A | FF FF /	FE | 3C219 | No access
6164A |	FF FF | FF FF | FF FF | C3 5A /	FE | 3C21A | No access
61652 |	12 5B | D8 5A | FA 5A | CD 5A /	FE | 3C21B | No access
6165A |	FF FF | 18 5B | FF FF | FF FF /	19 | 3C21C | The Stray Worldmap Level
61662 |	FF FF | 1E 5B | FF FF | FF FF /	11 | 3C21D | Hippo Level
------| Right | Left  |  Up   | Down  /	LB |  LBO  | Level name	
------+-------------------------------+------------+------------- -

Level bytes:
00 - 'Mushroom' Intro Level
01 - Tree Zone- Level 1
02 - Tree Zone- Level 2
03 - Tree Zone- Level 3
04 - Tree Zone- Level 4
05 - Tree Zone- Level 5 (BOSS)
06 - Pumpkin Zone - Level 1
07 - Pumpkin Zone - Level 2
08 - Pumpkin Zone - Level 3
09 - Pumpkin Zone - Level 4 (BOSS)
0A - Mario Zone - Level 1
0B - Mario Zone - Level 2
0C - Mario Zone - Level 3
0D - Mario Zone - Level 4 (BOSS)
0E - Turtle Zone - Level 1
0F - Turtle Zone - Level 2
10 - Turtle Zone - Level 3 (BOSS)
11 - Hippo Level
12 - Space Zone - Level 1
13 - Space Zone - Level 2
14 - Macro Zone - Level 1
15 - Macro Zone - Level 2
16 - Macro Zone - Level 3
17 - Macro Zone - Level 4 (BOSS)
18 - Wario's Castle
19 - The Stray Worldmap Level
1A - Turtle Zone - Bonus Level
1B - Pumpkin Zone - Bonus Level
1C - Space Zone - Bonus Level
1D - Tree Zone - Bonus Level
1E - Macro Zone - Bonus Level
1F - Pumpkin Zone - Bonus Level (2)
FE - No access
FF - Coin Wall

Level bytes provided by Cendamos (again).