hiking day trips from Paris?

You can easily walk 20 miles within Paris without ever leaving the city and remaining completed captivated by what you´ll see. 20 miles will require much of the day.

What’s your means of transportation, car or train?
Google Grande Randonée Ile de France and you’ll come up with the established trail system. This is a national organization withs thousands of kilometers of trails. For transport, we used to take trains and follow trails that took us between train lines: ie leave from one Paris station and return to another. The transportation options should be marked in the descriptions.

Otherwise, Fontainebleau and Meudon forests are two known places, Fontainebleau is known for sandy terrain, rock climbing, and horseback riding, in addition to hiking. There are other forests such as Compeigne, and grounds outside other local chateaux, such as Seaux and Chantilly. These forests have been cultivated and can be flat.

You can fill your backpack and mind with hiking books from Aux Vieux Campeur at 48 rue des Ecoles in the Latin Quarter.

You might get the book Half an Hour from Paris by Annabelle Simms. Here's the synopsis:

"Following the format of the small classic An Hour from Paris, and written with the same delight in the little-known treasures of the Ile de France, comes Annabel Simm’s latest guidebook, Half an Hour from Paris. Simms presents 10 new destinations easy to reach from central Paris, each with a carefully planned walk, ample meanderings through the cultural, historical and social milieu, comprehensive practical information and clear, detailed maps."

I got it for Christmas, have read thru it and her instructions for the walks seem very clear.

The grounds of Versailles Palace are extremely extensive and you could easily spend a whole day walking through the forests, wheat fields, and hidden gems of the gardens. Not really "hiking" but it's a beautiful place!