Act as a certified personal trainer with 10 years of experience specializing in mobility and functional fitness. Provide a detailed list of the [5 most effective exercises] for strengthening and stretching the hip flexors, tailored for [beginners/intermediate/advanced] fitness levels. For each exercise, include [step-by-step instructions], [recommended sets and repetitions], and [common mistakes to avoid]. Additionally, explain how these exercises benefit [specific activities like running, cycling, or sitting for long periods]. Use clear, actionable language and prioritize safety and proper form.