Hey @Jpacillo!  Welcome to CO.  I’m 17 years in now, but I remember when I first arrived in Boulder- there were/are a dizzying number of trail selections.  It would help to know what part of CO you are in or where you are seeking to hike.  One suggestion is Guanella Pass- especially beautiful this time of year.  The Mt. Bierstadt trail is easy and you can go out as far as you like- it does start at high elevation, but not a huge amount of gain unless you want to tackle the summit (it’s one of the easiest 14ers with only ~2800 ft in elevation gain. As someone else suggested, the Red Rocks loop is easy, with the distance you mentioned.  NCAR in Boulder has a number of great trail options right off the parking lot.  Chautauqua also has some good trails in the 3-4 mile range.  I can’t speak for other areas of CO, but in Boulder there is a New in Town hiking Meet Up group, so that might be a good option.  Happy trails!