Lolita,
Silicone breast implants, particularly Mentor's Memory Gel implants, are the most natural implants available. How they will look and feel, however depends upon a number of factors including: implant size, beginning breast size, placement over or under the muscle, and the type of capsule or scar tissue that forms around the implants. There are usually several options for patients considering this, and some may be better than others. However, the decision to go with silicone is a good one if you are wanting the most natural look and feel.
In most cases, silicone implants look quite good, and don't look fake. When you are running, however, you will want to wear good support in your bras in order to both support your breasts and maintain the shape of your breasts over the long haul. I usually recommend the UnderArmour "Endure" bra for women who have had breast enlargements (or who naturally have large breasts). They give great support and all my patients love them.
The best way to sort through your options is to spend some time with a board certified plastic surgeon and have an in depth discussion of this.
Best of luck,
David B.