I think this sounds nice for when you sprint:
-- You stop sprinting by slowing down - hitting a wall, letting go of the key, being thrown backwards by an explosion :D
-- You cant use anything (including items/blocks in your hand, blocks/items in the world. Exception for pressure plates, maybe chance of non-activate)
-- You auto scale blocks (your escaping so rapidly you just claw your way up blocks, sort of like a step height of 1 block. Maybe allow dashing over fences but also cancels sprinting right away?)
-- Your turning speed is severely reduced (Your focused on running straight forward. You dont have time for silly stuff like navigation. Maybe angle around the tree ahead of you, but anything else wouldn't be feasible.)
-- You cant jump at all (again, focused running. also means you cant get speed-boosted distance, or auto-scale up larger walls)
-- Either sprinting only lasts for a short while (5-10 seconds maybe?) or it can last indefinetly (just extending the penalty I mention below.)
-- When you stop sprinting you are slowed down for the same amount of time you sprinted
-- Post-sprint slowdown is of the same amount as the sprint speed-up (so if sprint is 40% faster, slowdown is 40% slower)
-- Speed-up and slowdown is BEFORE other speed modifiers (sprinting over ice is still faster, slowdown over ice is a crawl)
-- You cant sprint again for the amount of time you sprinted AFTER the slowdown (so sprint for 10 seconds, slowdown 10 seconds, cooldown 10 seconds)
-- Momentum! If you run towards a drop, you better hop its not steep :D (Note: Its never a good idea to run off a cliff)
A couple of extra ideas.... Armor could reduce the speedup (and increase the slowdown) by small amounts.
If your running at a Leaves block, you push through it, destroying it, and slightly damaging yourself.