As teachers are diving into Guided Math, many have found it has been difficult to turn their students loose 100%. They still see a need to do something whole group each day in order to make sure the students retain the concepts being taught.
Some teachers have found value in doing a SHORT, 10 minute mini-lesson, at the beginning of the math time, then letting their students go to their 2 centers, with each one being about 20 minutes long. Many 1st grade teacher like this approach because it has been difficult for their students to stay focused the entire time without other adult supervision.
Try a 10 minute mini-lesson, but you really should not do more than that as hands on small group instruction is better for ALL students.