Units that have Swiftness (X) have X% more Combat Speed than usual, causing them to move faster on the battlefield. Your other movements aren't boosted similarly, or at least aren't controllable enough to be useful in combat or anything else.

This is not to be confused with Quickness, which provides more useful buffs but generates increased fatigue.

Swiftness is a buff shared between Nature and Air:

• The Bless provides Swiftness (30), along with a +1 boost to Defense (DEF), and is stackable. It requires 3 on your Pretender.
• Three Enchantment spells provide Swiftness (50); these are Windrunner (a self-buff) at Research Level 1, Gift of the Hare (AOE 1+) at Level 2, and Haste (AOE 25) at Level 4. The first spell requires 1, the second requires 1, and the third spell requires 3 and 1 (or, if you're desperate for speed, 2 and 2).
  • The informative green text on those spells is wrong; they say they provide Swiftness (100).

• Some items grant Swiftness, too.

For stacking purposes, you should know that the three spells do NOT stack with themselves or each other.