public string GetData(string userName, DateTime fromDate, DateTime toDate)
{
 var userElement = xDox.Descendants("User")
        .SingleOrDefault(u => u.Element("Name").Value == userName);

var builder = new StringBuilder();

if (userElement != null)
{
    var result = userElement.Descendants("Attempts")
        .Select(a => new
            {
                Place = a.Element("Place").Value,
                Date = DateTime.Parse(a.Element("Date").Value),
                Distance = int.Parse(a.Element("Distance").Value)
            })

        .Where(a => a.Date >= fromDate
                    && a.Date <= toDate)

        .GroupBy(a => a.Place)
        .Select(g => new {Place = g.Key, Avg = g.Average(x => x.Distance)});

    builder.AppendFormat("User:{0}", userName);
    builder.AppendLine();

    builder.AppendFormat("Date:{0} to {1}", fromDate, toDate);
    builder.AppendLine();

    foreach (var item in result)
    {
        builder.AppendFormat("Average Distance in {0}: {1}",
                                           item.Place, item .Avg);
        builder.AppendLine();
    }
  }

  return builder.ToString();
}