The two standard forms are "a two-hour walk" [compound noun] "a two hours' walk" [genitive form of "a walk of two hours"] Google will find at least a hundred discussions on this. More information: https://theeditorsblog.net/2018/02/10/am-i-possessive/ [excerpt] Attributive vs. Possessive Nouns The choice between apostrophe and no apostrophe would be easy if the possessive was actually always truly possessive. But what we commonly call the possessive is actually the genitive case. The genitive helps us show possession, yet that’s not all it does. The genitive is also used to reflect measurement (including time)—five days’ worth of dirty laundry, a day’s drive, one dollar’s worth of penny candy, a month’s salary. We also use the genitive to show not possession or ownership but source. Consider this sentence: We all marveled over Natalie’s painting. Natalie’s painting could be owned by her, it could have been painted by her, or it could be a painting of her. If the painting was created by Natalie, the word Natalie’s shows source. If Natalie is the subject matter of the painting, for clarity we might write something such as we marveled over Marco’s painting of Natalie. But we could and do say Natalie’s painting, and given the context, others would understand what was meant.

'I took a two hour walk today' is correct - 'two hour' is the amount of time the walk took, so it is not possessive, but adjectival. Hope that helps!