Minecraft – eat in Minecraft

I suspect you used /gamemode 2 [player] instead of /gamemode 0 [player]. Gamemode 0 will take you back to survival, whereas gamemode 2 places you in Adventure mode, where no blocks can be destroyed, except with the right tools.

Instead of a number for the gamemode, you can use the full name (survival, creative, adventure, spectator) or an abbreviation of the name (s, c, a, sp) to avoid this mistake in the future.

You can boost a thrown Ender Pearl using a TNT cannon. Assuming you've got the trajectory exactly right and have a wall at the destination to stop the pearl, this is basically instantaneous other than the fuse time (4 seconds). Also, in one step you can only travel as far as the chunk load distance, which as far as I've read is 160m (10 chunks), or else the pearl will not land until you move closer. So, speed: 40 m/s average, less time to gather resources and place the TNT, and the flight time of the pearl (which will depend on the amount of TNT used and exact trajectory).

If you built a series of redstone-driven TNT magazines wired to operate in sequence and launched pearls perfectly to match them, then you could avoid waiting 4 seconds and move even faster.